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.
