The following information comes from "The Breech Loading Shotgun in America 1865 to 1940" written by Joseph T. Vorisek.
SPENCER GUN COMPANY. Gun maker located in Windsor,CA from 1868 to 1880. They were the manufacturers of the first successful pump action shotgun in the United Satates. The company was sold to Francis Bannerman & Company id 1889 and he seems to have continued to make the shotguns until about 1902. In addition to shotguns made by Spencer in Windsor,Ct and Bannerman in Brooklyn,NY it also appears a few were assembled by the English firm of Charles Lancaster between 1886 and 1888.
There is a serial number-year made table in the reference listing serials numbers starting with 001 in 1864 and ending with 21,000 in 1902. Give us the serial num,ber and we can tell you the year the gun was made and maybe by who. The serial number should be located some where on the receiver/frame. The gun was made in 12 gauge, chambered for a 2 1/2 inch shell with 26,29, 50 or 32 inch Twist or Damascus barrels. Because of this chambering and barrel construction, I have to recommend that you not attempt to shoot the gun but if you insist, please have it checked out by a good and competent shotgun gun smith before you do. And then use only appropriate ammunition which would be black powder and lead shot loaded shells. I would hate to see you injured and a old and valuable antique destroyed.