Well, I've hit the age where my eyes are no good with iron sights past 100 yards and I've been mounting optics on everything I can without screwing up the originality of the rifle.  That means that some of my rifles will mostly sit from here on out.  But I enjoy the RPD and I still wanted to shoot it.  Fortunately, the Poles made a specialized scope rail for it so I could mount one while still keeping the original look to the rifle.  In fact, it might even increase the value down the road if I'm lucky.  There isn't much on the internet in the way of detailed pictures or compatibility with combloc optics beyond a few "look at my rifle" posts and I could find zero in the way of official Polish documentation.   And so, I figured I might as well post what I've learned.  



Considering the fact that the RPD was designed as an open bolt, belt fed machine gun, you may ask why a scope rail was even designed for it.  Well, it certainly wasn't so that it could be used as a marksman's weapon!  Rather, it was intended for the NSP-2 night vision optic.  But it turns out that, when rebuilt as a closed bolt semi-auto, that Polish made and chrome lined heavy barrel is pretty darned accurate at long range; hence my desire to mount an optic.  



Alrighty, here's the rail close up and mounted:



This rail was Polish military issue.  I'm assuming it was mated to an RPD at some point because there is a serial number with a letter prefix electro penciled on the back of it.  There is also a number faintly scratched into the bluing on the face of the rail but it's hard to pick up with a camera.  You might just barely be able to make out an upside down "139" in the picture above.  I'm assuming that was the scope number but that's only a guess.  



Here, we can see a lightly struck inspection stamp amongst those finely crafted communist milling marks:



I say that tongue in cheek.  This thing is very finely made where it needs to be, it's rock solid in construction, and there is zero play in the components.  



I don't know if this rail was originally a Soviet design or home grown but whoever thought it up was pretty clever.  When there is an optic mounted and ready for use, it's impossible to open the top cover on the rifle so you'd think reloading would be impossible.  What you do is pull back on the curved rod sticking out the top rear of the rail and it swings down, allowing you to lift the top cover on the rifle for reloading.  Here, we see the rail swung away from it's base which is firmly attached to the receiver using three screws:



The angle of the photograph makes it look like there is a rod sticking out the rear of the hinge but that's just the rifle's cleaning rod which mounts along the side when not in use.  Sorry about the confusion.  Anywho, this rail doesn't just fall down when you pull the release rod.  You have to pull it down.  They've made the hinge a precision fit so that the scope will hold zero.  There is absolutely no play in the hinge.  As I said earlier, this thing is finely made where it needs to be.  Notice that there are three screws holding the base to the receiver.  They extend all the way through the receiver and you can see them from the inside, but they are flush so as not to interfere with the bolt as it travels back and forth.  Judging by the way the ends of the screws looked when I received this, I think they were peened in place originally.  Thus, the ends needed to be dressed a bit before reuse.  They are not peened now because this rail covers the serial number so it needs to be removeable.  Currently they are just tightened down but I'm sure a bit of blue locktite will do the job should they loosen in use.    Notice too that there are two pins, one at the front of the base and one at the rear.  These two pins are essential as they take all the shearing force under recoil.  The screws themselves do nothing more than hold the base in place. in fact, they are of a significantly lesser diameter than their holes in the base.  The pins are what keeps the base from moving around and they fit both the receiver and base so tightly that you need a hammer to separate the receiver from the rail after the screws are removed.



There are also inspection marks (and some cosmoline I didn't remove) on the base:







Notice the number "196".  This is a different number than the one scratched on the rail.  I don't know the significance of either three digit number but I'm pretty sure they are not inspection marks but rather a serial numbers of some sort.

As I said earlier, the whole idea was to be able to mount the NSP-2 night sight.  And, in fact, the NSP-2 slides right on and locks up perfectly:



The problem is, most shooters don't have one of these laying around and, while it can be used during daylight hours, saying that it's impractical for a day at the range is something of an understatement.



The 3-9 power 1p21 slides on but you can't lock it down because there is a cam on the locking lever axle that prevents swinging the lever to the locked position:



It could easily be made to work by milling a notch for the cam into the scope rail but I'm not one for permanently modifying original vintage equipment.



However, if you were to cut the required notch, other neato Soviet era optics could be used.  Some of these include:

The 1pn34:





The 1pn58:





The 1pn51:





And a bunch of other neato optics that I don't have.  Pretty much anything that will mount on an AK style rail would fit this rail.  Just remember that, for some of them, you would need to mill the aforementioned notch.



Having looked at some of the more weird options, let's consider the most practical option, the lowly but venerable PSO-1:



It slides right on and locks down with no modifications required and the eye relief is fine.  You don't get much of a cheek weld but you just adapt.



The scope is slightly offset to the left.  I don't know why, but I think an offset scope is neat looking:





Earlier, we looked at the rail swung down from its base.  Here, we see that in practical use with the PSO-1 attached:







Finally, a shot showing the top cover raised and ready for loading:





After reloading, you simply lower the top cover, swing the optic back into position and you're back to shooting.



Something I didn't mention earlier is how you know where to mount the rail base on the receiver.  While I haven't found any period documents showing exactly where it is supposed to go, it's idiot proof and I should know because........



Anywho, the height of the base is very nearly the same as the height of the receiver so you just center it vertically.  As for where to mount it horizontally, you have very little play.  At the rear, you have to clear the takedown pin in the receiver.  At the front, you have to clear the protrusion on the left side of the top cover.  Taking these two factors into consideration, you have so few millimeters to play with it's inconsequential.  So, it's pretty much impossible to screw it up.  



So, now I can take my RPD buddy back to the range again.  YaY!  And now there is more information on the internet than ever before about this obscure RPD accessory.  YaY!  



Okiedokie, that's it.  Thanks for your time and I hope you found some enjoyment in my rambling.  I love you Mom!

That is cool and is something I never knew existed.
Thank you for sharing!

Very nice,wish I had a mount like that.
I looked at buying a separate top cover when parts were not so expensive and putting on some home made picitiny something but never did.
First time I've ever seen your type mount.
Nice write up,let us know how it shoots.

They are excellent mounts~