DD

QUOTE FROM ABOVE "The action you are looking for should have double underlugs and to
be ideal have chopper lump barrels. You also will want a third fastner like a "Greener" cross bolt
and it would be nice to have side clips on the barrel."

The above is really a description of an Anson and Deeley Boxlock Shotgun such as a
GREENER Pidgeon Gun or many of the other English SxS Shotguns.
I say Pidgeon guns as they were proofed for 2 3/4" cases and had Pistol Grip stocks.

I think the following may fit IF you can't find a Greener
BSA
21 Midland
27 Simpson
28 J.P.Sauer

German guns were always strongly built.

500 Nitro