Hi Bob!
I’m afraid that I can’t all five question but here’s my best shot:

1) 3/8” long by 1/8” diameter
2) Yes
3) ? Don’t have a pitch gauge but very fine machine threads. I’d guess 40tpi.
4) slotted. The head is cylindrical, flat headed and polished & blued to match the M41’s finish.
5) yes. If you have a time machine, It’s #6511 muzzle break screw $0.10 in 1959.