I would imagine it to be home-made by someone, in simple terms a "Zip Gun". It is interesting in that it appears quite crudely made on one hand, and over-built and overly complicated on the other! That firing pin looks like an automatic pierced primer if the "thing" was ever fired!
What is the significance of the numbers on the note shown in #3? Do you know what it is chambered for?
With it appearing the back of the trigger finger would be about flush with the muzzle I doubt anyone would want to shoot it more than one time, it would hurt!
Do you have it in your possession, or just photos? I bet the ATF would be interested.