Can anyone identify this?

[Cofaler]

Is it time to play Let's make a deal?

Maybe.... The adapter came on a 21" rifle sight 'S' dated barrel, which I assume is the barrel the adapter was on originally. A buddy bought it for $13 at a yard sale and gave it to me since he knew I was into old shotguns. I didn't know what it was either, but did some research and found out what it was, and what it's worth. Yes, I told my buddy and will compensate him fairly. Now I'm gathering the rest of the parts to build a Mk1 clone, and I do need a mag extender, vintage with some patina.

Like was noted earlier, the bayo adapter and plain fore end are the tough pieces. I started to put it together piecemeal, but it looks like it will be cheaper to buy a donor gun.

This $13 gift is going to cost me some coin.... Heck, two weeks ago I didn't even know the
USMC used 870s....

Did these come parked originally, and what finish is correct for the trigger assembly and bolt?