Thanks, gents. Pathfinder, I really appreciate the measurements that you sent.
Coffinmaker, I pulled the image from the same eBay ad that you found. I was just using it to illustrate which kind of screwdriver I was talking about. You're right--at this point they are of rather little practical use. I was hoping to have something that I could use to make sure that none of the screws were getting loose while I was at the range, but for any real work, my Brownell's screwdrivers will be used. Mainly, though, I just think that this kind of stuff is interesting.
I'm attaching a photo of my first attempt to make one of these. I marked it with my Celtic cross stamp, although I doubt that anyone would have taken my product as an original; I am not the craftsman that many here are. Note that it is hollow ground to fit my guns' screws. With this attempt, though, I made a fundamental mistake. I'm among friends, so I'll admit to it. I had thought that I had grabbed a piece of high-carbon steel from my scrap bin. I should have realized that it was low carbon by how easily I was able to work with it. Just after I took the below picture, I attempted to harden the piece in preparation for tempering it. No, it didn't harden. It is way too soft to actually be used, so I now have a template/practice piece for my next attempt. Oh well, it's not like I have plans to go anywhere these days...
Again, I appreciate all who have added to my efforts here!
CC Griff