Seems like whenever people mention getting a Ruger they always say the Mark II and then the Mark III.

How come people don't mention the Mark I?
Is it because no BHO?

Otherwise it seems like the same gun as the mark II (Take's the 10 round mags also).

The Mark I is a fine pistol; always has been.  Some design changes make the Mark II & III's better.

In addition to the bolt hold device, they also changed the magazine release location. The MK-I's have the mag release on the bottom of the grip, while on the MK-II's & MK-III's the release is located on the side of the frame. I am not sure if the magazines are interchangeable due to this....

ETA:

The newer models are also drilled and tapped for scope mounts too....

My MKII has the mag release at the base.


Here is a link to a nice article with a lot of info on the MK series.


http://xavierthoughts.blogspot.com/2007/12/ruger-mkii.html

Personally, I prefer the Mk I's.  They generally have better triggers since they hadn't started monkeying with the internals to add slide hold opens, and relocated magazine releases.  The Target Model is more accurate that all but the best shooters can shoot.

The bold part is partially incorrect.  The MKII (non-22/45 type) has a heal mag release, just like the MKI.  

Yeah, the 22/45 was the one I was familiar with. I am not sure why Ruger would make only that version of the MK-II with the button mag release and the rest with the heel style release....

I prefer the button style release myself and I also like the fact that the MK-III's come drilled and tapped for scope mounts.

I actually kind of prefer the Mark I's.  The safety doubling as a slide lock/release is handy and efficient (safety concerns aside).
But what I don't like about the Mark II/III's slide release tab/lever is that it's a real flimsy-slim thing that is jammed up against the grip and sometimes a pain to activate.

Probably because the Mk II has been/was around for a long time.  A lot of shooters have probably never seen a Mk I.  Switch was made, when, 80's?

I have owned this Mark I since the fall of 1981.  Thousands of rounds downrange and still superbly accurate.  Feeds whatever ammo I stick in the magazine.  One of my best firearms purchases.  

I now have 2 Mk1's with the 5.5" bull barrels.

I bought the first one in about 1980.  Used of course (I seldom buy new guns).  I had a Ruger Single Six and my younger brother wanted the Mk1 so I told him okay, he could buy it off me.

We took it out to shoot it the first time and he wanted me to sight it in for him.  I put up a target at about 15 yds. (you know sometimes sights are way off and if you don't start up close you won't even hit the paper) and took the first shot.  It hit about 3" low and 2" to the right of the point of aim.  I decided to take a second shot at the same POA to get a group going before moving the rear sight.  After the second shot I just could not believe I missed the whole target.  I turned, looked at my brother and shrugged.  I took the third shot and that hole sort of looked funny.  It suddenly wasn't quite round anymore and I began to doubt if it had been round.  I took the fourth shot and the hole got a little more ragged.  I turned and looked at my brother and started to get excited about how that pistol was shooting.  Now I knew the second shot had gone through the hole the first shot made as had the third and fourth shots.  I took that 5th shot and it was just slightly outside the hole the other four had made.  I grinned at him and asked if I could buy it back and he turned me down.

I traded my single six in on one about a year later and it is also extremely accurate.  When my brother passed away 3 years ago I got that first Mk1 back and gave it to my youngest son.  He fell in love with it on our first shooting trip.  He was taking out butterflies on flowers at 25 and 30 yds. from the standing position with two hands on the gun.  I told him not to tell grandma when we got back to the house.

My mom would be pissed. She is a certified Master Naturalist and Master Gardner. Killing butterflies is blasphemy to her.

Love all my children equally