A set of Belgian made bbls (aftermarket) in 12ga and fitted to what was orig a 10ga H&R ,,,maybe,,just guessing.

They made these is 10 & 12ga.
That step down shoulder on the bbl from the frame is what I noticed.
Was that done to make a lighter weight set of bbls to fit the 10ga frame maybe.
I don't even know if they used 2 different frames,,just guessing as I said.

I have a 10ga example of one of these. It's apart right now for making a new mainspring for the RH side. That spring has a slightly shorter lower arm and looks like the shortness was from
breakage. It still worked OK, but a new spring is right for it.
A lower grade than this one with very little engraving. I'll pull the parts aside and see what marks are on it as far as proofs and such.