It is an S&W No. 1 1/2, First Issue. They were made from 1865 to 1868. They made 26,300 of them, so they are not extremely rare, though not common. In average condition, they bring about $300-400, twice that or more in excellent condition. From what I can tell from the pictures, that one seems to be on the lower end. About 6 were not up to S&W's normal standard of quality and are marked "2ND QUAL'TY"; those bring a very nice premium, but there are counterfeits.
(FWIW, I can tell from the square butt and the shape of the grip where it meets the frame. The No. 1 First and Second Issue are .22 caliber and have a straight edge at the top of the grip; the 1 1/2 Second Issue has a birds head butt, and the No.2 also has a straight edge grip.)
Jim