It looks like a pen flare initiator however there's a pocket clip on it, which I have never seen. If it is the initiator, then it will shoot pen flares and set off shot gun primers for you. They're great for initiating shock tube/NONEL. Looks a little old though :)
Yeah I noticed your trigger haha. Nothing but a spring loaded firing pin inside of a case. Simple and effective. Have fun! Make sure plenty of alcohol is involved :)The "knob" half way down is the trigger, found it at my dad's;so no telling how old it is. Guess I need to find some flares to play with (HH6 will shit a brick on this, LOL).