I had a couple of top covers with the screws, and quite a few more with the pins. I always thought the screwed versions were the original type, and the pins came later because the screws tended to loosen and get lost. Now I'm not sure, as the 50 BMG uses twisted wire to secure screws on the RSG, mush like the pincs hold on the top cover. So maybe the pins version was the original? Anyone know for sure? Dolf's book must have something on that listed somewhere....
The originals are the one with the screw. There is a headed pin that slides through from the top, and is secured by the set screw. That is the USGI assembly. Sometimes a lock washer is used as well.
The type we see on the Izzy kits, with the taper headed pin inserted up from the belt feed lever and pinned over a washer, is an Izzy modification. The worst part is the cut a chamfer in the belt feed lever to interface with the pin's head.
My guess is that this was done to overcome what some perceived as a drawback to the original design. That is, after a little wear, the lever can droop down a bit when the top cover is open, as it is held only by the fit of the pin through the hole. It does not affect function, but it can require one to watch that the lever is in its correct position when closing the cover. Doesn't bother me, and I don't like the Izzy design, but I understand why they did it.