[napp]

Thanks for all the replies, guys.

First of all, I'm convinced that those of you who say it isn't the original screw are correct. I've surfed the Internet for close-up pictures showing the screw on other GPRs. All those that I found show a slotted head rather than an allen type receptacle.

I can't find anything that defines the length of the original screw; but I'm betting the one in my rifle is longer. I did some checking for interference with the nipple (as mykeal) suggested).

If I remove the nipple, the clean-out screw will insert fully. Then, when I install the nipple, the bottom edge of the nipple will contact the clean-out screw and stop short of full insertion (probably two threads short).

If I reverse that process and fully seat the nipple first, then the bottom of the clean-out screw will contact the nipple base and stop a few threads short of full insertion.

I suspect that the original owner lost the standard screw; and replaced it with a set screw which had matching threads, but was fractionally longer. Apparently, his solution to the interference problem was just not to insert the screw deep enough to contact the nipple base.

The rifle fired fine without issues with the non-standard screw; but I went ahead and ordered a correct replacement from Lyman earlier tonight.

Once again, thanks for everyone's help.