I've seen couple of online dealers claiming this 9 1/2 inches of cap gun coolness dates back to 1940. I don't buy it. Looks like late '50s to me.
This is what I'd call a double single-action weapon. Each hammer must be cocked separately, but a pull of the single trigger fires both barrels.
Caps were loaded by sliding them into these slots beneath the hammers. I've never seen square caps, so I'm guessing young pirates were expected to tear off short lengths of roll caps.
A bunch more cap guns HERE.