The Solid Rib feature is a key to extra value placed on the OP's inquiry.
A plain bbl Field M12 in 20ga @ 85% would be a $400 to $500 gun most places.
Add that SR bbl, and to a 20ga, and it increases the value of an otherwise plain M12.
85% condition is no where near most collectors demands. It's nice, but it puts it in the 'shooter' catagory IMO.
Don't pay premium.
The suspect replaced butt plate may be just that, or it may be a replaced butt stock w/that plate from a later M12.
It may be a repro butt plate too that was used to replace a broken orig.
A repro will have a different look to the material as they are cast out of black epoxy in most instances and small, sometimes near pin point air bubble inclusions can be seen on them. Expecially on the edges where they were fitted to the wood.
Orig butt plates were screwed down in place and the butt stock machine sanded (held by hand) lengthwise on belts. So the sanding marks (grit marks) extend right from the wood and on to the edges of the butt plate material. No high or low spots except perhaps from a bit of 70ys of some wood shrinkage.
Refitted plates, new or repro, are usually easy to spot as they are done as careful as possible so as not to touch the orig wood finish.
That leaves the plate a bit high or sometimes low if done off the gun. Grit or even file marks often go around the plate instead of with the grain/length of the wood as a factory fit would do.
Touched up wood finish around the rear edge of the stock at the butt plate is another clue to a refitted plate.
If the plate looks orig and the fit looks orig to the butt stock,,then I'd suspect a replaced buttstock w/ butt plate if the era for the plate is wrong as you think it is..
Again, originality is everything when paying bigger bucks , especially for M12's. There's a lot of them still around out there.
If you really want one and it's to be a shooter. That gun sounds like a good candidate to me.
But the price is way out there for the stated condition and possible issues away from originality that I think you are describing.
I would think the gun would be in the $600 range to sell. Maybe a bit more if someone really fell in love with it.
added..
Also check if orig,,ser#'s match on both assemblys,,no signs of a reblue on anything, T/D adj is still in it's first or second take up position and not near the end of it's usefullness, Make sure the SR is secure. It's soft soldered to the bbl and while it's not very common for them to come loose, they do on occasion.
If you can, check functioning with dummy rounds.
because yes, they do wear out and need repair some times.
