I found this one broken off inside a rifle by William Schreckengost. I think it somewhat unique. It threads on one way for use and threads on the opposite way for storage.
Missed the bit about the rifle at first, too busy looking at the worm...What was the caliber of the rifle and the size of the worm? The one I have is about 1/4" wide for a 20 gauge shotgun, and seems to small in proportion to the bore to hold a wad of tow tight enough to the barrel to get down inside rifling. I'd want a worm much closer to bore size if for a rifle, I'd think.