You old shotgun is a popular back-action style lock that began with the French M.1840 infantry musket. The design spread over the next 20 years and became popular with gun makers both here and in Europe. maker's marks or any proofing or inspection stamps can be illusive sometimes. Check under the barrel breach or even underneath stock hardware. "Viewing" and "inspector's" marks can show up in some really odd places. Maker's marks are usually engraved on barrel or lock plates but age and ear can virtually remove these if not deeply cut. Sometimes holding a light at odd angles to the metal surface will enable you to see them, even if worn too much to actually read.
As mentioned above, these old back-action guns generally don't bring a lot of money but a Christmas goose, or even squirrel, harvested with one will taste extra yummy! :wink: *