“Part of a sight maybe ?”
Thats going to be the base parts to the rear sight for either a FN FAL or a L1A1 Brit FAL. It’s difficult to determine exactly which one due to the lack of the sighting leaf assembly, but it’s probably for the FN FAL.
This particular one was also owned by CSN Admiral Raphael Semmes.
https://laststandonzombieisland.com/tag/36-cal-houllier-blanchard-revolver/
You’re next now…
Foote‘s Encom company made some pistols using the side feed mounted m3 greasegun magazine. Foote even made a .22lr mini copy of the Sterling smg side feed in a semiautomatic pistol. You can see the resemblance to the photo that was posted: