I think it's for removing the base pin... Pretty sure it's basically a flip-up handle that unscrews. At least that's something I've seen on another old single-shot shotgun.
EDITED TO ADD: I think yours is just like this one, though this one is hammerless:
I have that exact same shotgun I believe , I inherited it years ago, belonged to my grandfather, I haven't shot it, it hangs on the wall in my office at home, when I get time I'll take it down and try to take it apart, I did some research on it years ago, but don't remember what details I managed to find out about it, I know it wasn't made for modern shells
I had one of these. Sorry I don't have a good close-up picture of the right side of the receiver where the takedown pin had that flip up around lever that looked a lot like a nickel.
My shotgun was certainly made by Davenport: however it was only branded with some local hardware store or sporting good store's name: "Etowah."