I wish i knew more to help you; but it's above my pay grade. perhaps someone more informed could help with identification. #6-48, and #8-40 are some of the most common gun screws. i don't think they are used much by other Industries, and they're the most likely. I can't remember if the Tang screws are the same as the bolt stop/trigger spring screws, but the Tang uses #8-40. #6-48 are used mainly for holding down scopes.
Take the Tang screw out, and see if it will work in the hole(s), than you'll know it's an #8-40.