Yes, the receiver is a real military riot gun. It's one of the early block of Stevens riot guns, and some long-barreled training guns, that were made pre-WWII. Hard to find these.
Seller is asking $2500...
I was totaling up parts value vs cool shooter value and came to around $600-$800 as well. The heat shield, if real, would be the most valuable piece. Not convinced that it's real, it could just be sun/solvent faded, hard to tell at this point from just pictures.
James,
I'm a collector and researcher of Stevens 520/620 shotguns.