The methods above may work well. Since the threads are going into steel, you also might be able to put a taper on a piece of soft copper wire that is close to the correct O.D. and screw that in far enough to get an impression of the threads. Then you would use a screw thread pitch gauge to determine the correct thread size. I wouldn't be surprised to find it is not a modern standard size. I think the original screw had a pretty large head. I believe this forearm latch arrangement carried through all of the later rod cocking models, so you should be able to find a replacement from Gun Parts Corp., or other gun parts sources.
