Looks like it started out as a centerfire .410

I have a cape gun very similar to it that started life as a .22LR on one side and a 9mmRimfire Shotshell on the other, another one that's 9mmRimfire Shotshell and a larger rimfire rifle round. both of them very similar in appearance to that little gun's action.

Cute little critters they are, the .22 that I have is silent with CB Long ammunition from CCI and amazingly accurate.
I've took about twenty squirrels with it last year.
The 9mm shotshell is great for very close range against squirrels and pest birds, and will absolutely tackle mice and rats.

Perhaps I'll have to get photos of them up here some time.

--Tinker