Don't think this is a firearm but it is a Flare, Signal, Red. I carried this particular one for a year in Vietnam and smuggled it back in my hole baggage. The flares came in a plastic holder of 6 attached to the pen gun with a piece of light paracord and we attached the whole thing to our flak jacket. Made by Penguin Industries in November 1968, these screw in types are much more reliable than the more current press in variety, in my experience. I often see the older flare pistols but rarely these. We sometimes also carried a handful of 38 special tracer rounds for the issue revolver in case we needed more signaling. Remember upcoming Veterans Day!!
