NOT very old. First clue is that it says Japan---if it said anything
at all an older gun would be marked Nippon.
It looks like a kit gun, probably no older than the 1960's. If the pictures
are correct and the lock is on the left side, it's what's called a horse
pistol, pommel pistol or saddle pistol. They would be carried in pairs, in
holsters slung on either side of a saddle in front of the rider. One would have
the lock on the left side, the other on the right side.
I'm guessing that the bore measures real close the 5/8" and it is a
smoothbore? A lot of the kits were sold as a "Tower Flintlock Pistol".