I had a Remington 870 Police folding stock that had the cut out for the rear sight. The Remington version had the sight on the barrel. The stock was painful to use. When I recently found a good deal on another old 870 Police with the folding stock I snapped it up. I did not however fire the gun till I took that stinky stock off and put some old wood from a pawn shop special on it.
If I recall correctly the Remington stock was pretty much open along the top and should handle a receiver mounted sight in most cases.
I believe Choate has a similar model with cutouts if you can find one to check.