Well, I made the decision today at a small shop to buy my first .22lr pistol. It's a Ruger Mk I 6" tapered barrel. I was looking more into the Mk II, he did have one, but it had a scope mounted and the normal sights removed. I went with the Mk I with adjustable rear sight. When I looked it over, it looks like it's never been fired. I'll post pictures in a few days. Since I'm in NY, i have to go do some paperwork before I can pick her up.

Good for you!
You'll love it.
Don't fall for the "I sold the dang thing/I pulled my hair out/I said bad words, because I couldn't get it back together" BS.
Watch a couple YouTube videos on disassembly/reassambly and don't over think it and enjoy.

I heard about the "stand on your head, on the full moon, do two shots, and pray to Odin" to reassemble. I figured once I get the hang of it, it won't be an issue. I'm going tomorrow to do the paperwork and Saturday I'll get it and post pics.

I have the same pistol. Technically it isn't a Mk I, just the Ruger Standard Model. When the Standard Model was improved, the results were called the Mk II, Mk III and now Mk IV.  

I recently picked up a MK 3 22/45 and love it.  Since my gun is new and tight I have a hard time getting the vertical pin of the mainspring housing to all the way through the receiver.  Lining things up to get the swinging hammer spur in the right spot is easy.  Youtube videos are your best friend for this.

Pictures as promised



And the other side:



When I picked her up Saturday, I also got a brick of .22lr SV and a MKII mag that I've already switched the button over to the other side. I found a chart online that makes it look like she was made in 65.

Good looking pistol! Never had the pleasure of shooting a tapered 6".
I'll bet it has a good balance to it.
For some reason, the rear sight doesn't look factory to me. Wonder if I'm mistaken and it's just an older, original style or if it has been replaced with an aftermarket?
Hopefully, if you haven't already, you'll find some time during the holiday to get to the range with it!

Technically, the OP described the RST-6, the "Standard" model pistol with the 6" lightweight tapered barrel.

Which made 10mm's post accurate.

But what the OP has is instead a T678, the  Ruger Mk I Target with 6 7/8" heavy tapered barrel.  


The six digit 321XXX serial number places it in the 1965 production year.


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The Mk I added an improved trigger that was a little less gritty, and it added the "Micro" adjustable sight and an undercut from sight.

The trigger and sight however were options on the Standard, which basically made them 4 3/4" and 6" light barreled Mk I "Target" pistols.

Wood grips were also options from the factory.  By 1971 however when Ruger switched to the A100 frame, they were just after market options.  Similarly, the Mk I adjustable rear sight and Mk I trigger were no longer factory options on the Standard.


The Mk II added trigger stops to reduce over travel and slack, a bolt hold open device and scallops in front of the cocking handle.  The Mk II was arguably the peak in Ruger semi-auto pistol evolution.

The Mk III is where it started going wrong when  Ruger added a  magazine safety, although you can get after market hammer and trigger kits to convert it back to Mk II status.

I'm going to wait and see on the Mk IV, but so far I'm not impressed and I'd pay more for an excellent condition Mk I or Mk II than I would for a new Mk III or Mk IV.  



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You won't miss the bolt hold open, and the 10 round M-10 magazines from the Mk II pistols work just fine in the Mk I and Standard pistols.   Given the logo on the left grip panel, your's is a pre-1971 "old model", but you just need to move the button on the Mk II M-10 magazine to the other side to use it in your pistol.  It takes less than a minute to do that.


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Disassembly:

1) Clear the pistol to ensure it is empty.

2) Pull the trigger to be sure the pistol is uncocked.

3) Use the curved end of a small paperclip to hook the lever for the mainspring housing.  This avoids the scratching that occurs with screw drivers, etc.

4) Pull the mainspring housing and bolt stop pin assembly out.

5) use a soft rubber mallet to lightly tap the receiver forward to disengage the frame.  On new pistols or little used pistols a hammer is needed. Once it's well worn you can probably push it by hand.  

6) Pull the bolt out of the receiver.

7) Be sure not to lose the cross pin in the bolt that serves as the firing pin stop.

If you lose this or re-assemble the pistol without it, and you dry fire the pistol (even once) with the stop pin missing, you will put an impressive dent in the face of the trigger and you will have metal intruding down into the chamber, making it impossible to chamber a round.    This is a big deal with the Mk 1 and Standard pistols as there is no bolt hold open and you will pull the trigger on an empty chamber from time to time if you lose count of rounds fired.

They make a small tool to "iron" this dent back out, but you want to avoid this as it can quickly result in needing a new barrel and that's very expensive to replace. Ruger will not do that particular repair and you'll need to buy a new after market barrel and find a gunsmith to swap it out.  

8) Lift the recoil spring off the top of the bolt.

9) Remove the cross pin in the bolt and carefully lift the firing pin out of its groove.  Note how the spring and guide rests in the groove (hook down, guide forward).


Re-assembly

1) Lay the firing pin spring back in the groove properly oriented hook down and guide forward

2) Lay the firing pin in the groove and press forward enough to insert the cross pin.

3) put the recoil spring back on top of the bolt.

4) You can install the receiver on the frame by lightly tapping the muzzle with a rubber mallet, or by resting the muzzle on a padded surface and pressing down with the receiver.

Getting the bolt and mainspring housing back in isn;'t hard, but order and hammer position are important.

5) Line up the hole in the frame and receiver and verify the bolt stop pin can be fully inserted, by fully inserting it, then remove it.  Tap as needed to fine tune it until the pin will press in.

6) Pull the trigger, tilt the pistol muzzle up and ensure the hammer is all the way back so that you can insert the bolt

7) Insert the bolt, making 100% sure that the cross pin that serves as the firing pin stop is present.

8)  Hold the pistol muzzle down, pull the trigger and shake the pistol a bit to get the hammer to fall all the way forward again, so the tail on the hammer is free of the cross pin in the frame.

9) Insert the bolt stop pin all the way, and then tilt the pistol muzzle up so that the tail on the hammer falls back against the bolt stop pin.  This lets the end of the tail rest against the cup for the hammer spring.  

10) Keeping the pistol muzzle up, latch the mainspring housing.  If you've got it right you should feel some pressure when the spring is about 1/8" to 3/16" short of being flush with the grip frame.  

11) Cycle the bolt to make sure it functions, check the trigger and check the safety function.   On the Mk I and Mk II it should only go on safe when the pistol is cocked.  The Mk III can be placed on safe when cocked or uncocked.


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Both my Mk I T678 and my Mk II 5 1/2" Bull Barrel function fine with both high velocity and standard velocity ammo and they will generally shoot 1" groups at 25 yards with standard velocity match ammo and around 1.5" to 2.0" at 25 yards with good quality high velocity ammo.  If you're not getting that lind of accuracy with your's blame the shooter not the gun.

You should avoid the lightweight, longer cased hyper velocity ammo in all the Ruger Mk I, II, III, and IV pistols as using it will result in the bolt stop pin getting battered.

The design lets you clean the barrel  from the breech end so do that to protect the muzzle.    Also, there's no need to clean the barrel all that often and any more than every 1000 rounds or so is probably too much and will; produce more wear than shooting it.  These pistols will shoot well over 100,000 rounds before they wear excessively, just be sure to replace the recoil spring every few thousand rounds and keep the bolt and inside of the receiver clean and lightly lubricated.  


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Note the Ruger emblem on the right grip panel on the T678 below.  It's a 1981 model and as such a "new model" with the A100 frame.  The pre-71 pistols use the A1 frame and have the emblem on the left grip panel.  The hole spacing is different for the "old model" and "new model" grips so you can't interchange them.  That makes the emblem position fairly definitive for frame type, but the Mk II has an emblem on each  side.

My first .22 handgun was a Ruger Single Six.  

My second was a Ruger Mk1 5.5" bull barrel.

After the Mk1 I had no need of another .22 handgun.  Darn thing shoots like a short barreled rifle.

After I got mine and family members saw it shoot and shot it both my dad and my youngest brother got Mk1's of their own.

This was over 30 years ago and all those guns are still in the family even though my dad has passed away.

My younger brother had one (like mine but older) and when he passed away I gave it to my youngest son.

His first day shooting it he was knocking down butterflies out past 25 yds when they'd light on the stick weeds to feed.  Yes, I told him not to go back to the house and brag to his momma about all the butterflies he killed.

They don't necessarily need match ammo to shoot awesome groups.  Mine was very fond of Remington Thunderbolts (this was back in the early 80's, that ammo has a bad reputation these days) and was both reliable and accurate with them.  Me and another college buddy (he had a High Standard target pistol and shot Eley match ammo) used to go squirrel hunting with our .22 pistols.  Headshots with a pistol, that's a rush (sneaking up close enough to get a headshot that is).

I think you can still buy Mk1 magazines, but if Mk2 mags work, why not.

Finally got to the range earlier today, ran 200 rounds of CCI SV through her. The MKII mag worked perfectly with the button switched. I ran into some problems with the MKI magazine though. There were two lips at the top of the mag that are supposed to keep the round in, well they were so tight it was rather difficult to load, I think they also caused a couple feeding issues. I'll try messing with the lips but I think I'm going to buy a few more MKII mags.

I believe the original Standard models had fixed sights but the adjustable sights pistols were Mark I when they were introduced a year or so after the Standard.  Design changes became the Mark II, III and IV.  The photo nails this pistol as a Mark I.

Nice pistols.  

My first gun was a Ruger Single Six, on which I learned to shoot handguns.  I sold is some years later.  Not long after buying the Single Six  I bought a Ruger Mk II Target and still have it, great gun.

I had one but in a weak moment I sold it a few years back.

The first time I disassembled it I had some issues getting it back together, but once I figured it out, it was not a problem.

All I've got left is the picture

Bought my MK 2 in about 1982. Awesome gun. Can be tricky to reassemble ,but mine has gotten loose enough to snap back together in just a few minutes.