Does anyone have any idea of what the heck I've got here?

[Hellraiser]

I think Frank 46 nailed it. The receiver looks like my Husqvarna 16 ga.

Hah! And...

If ya can't shoot... just put some gas in it... and saw up some trees with that Husky.

 

[old bear]

I was remiss in not mentioning this before. The bored hole in the sideplate, was for a pin that held the hinged loading gate/breach. When the hammer was cocked the loading gate/breach would rotate up to allow a round to be ejected or loaded.