I have one that looks almost exactly like yours, just a difference of a plate and a couple pins. Hammers, triggers, stock all look exactly the same. On mine the left side of the receiver is stamped Hopkins & Allen Arms Company, SN 40480. Serial number on mine is under the breech, you have to break the action open to see it.
Google shows H&A shotguns made early 1900s, most of what I saw were hammerless.