I think you have the New Model Target Pistol. It was made on the H&A large topbreak revolver frame, and came in several barrel lengths. It was made for BLACK POWDER cartridges only and will not hold up to modern ammo - there are NO replacement parts for them. Check this site:
http://iverjohnsoncollector.x10hosting.com/for Bill Goforth, the expert on H&A's - give him your pics and he will tell you all there is to know about this nice old gun.
IIRC, H&A went out of business around 1915 after a fire in the factory - no records are left of SN's, etc...
Great old gun.
mark