Is the date stamp just a patent date?

So I will offer a little family story that will cringe every member in here. My great grandfather was the last to work at the factory and was a wood scriber and a machinist. He Personally built his own .410 and had it for years until eventually trading it in to pay a bar tab in the late 70’s. The family never was able to trace its path. The city of Oswego were he lived had and has a lot of bars. My grandfather said that his fathers initials were on the inside of the forearm. FS or FES. Sooooo If anyone has a late year .410?