Belt Feeder,
What you have is a Stevens Model 620 (actually a 620A) made in early 1942 for the US Military in WWII. Your shotgun is one of the first batch of Model 620s for the government, they all had the US property, ordnance bomb, Col Gilbert H Stewart (GHS) stamp and proof "P" on the left side of the receiver. Your shotgun was originally a long barreled training gun (for training aerial gunners) because of the Imp Cyl Choke marks (***), but has been cut down to simulate the appearance of a riot gun. The smooth wood is correct for later military production and is probably a replacement during the war or during an arsenal rebuild later (the 620 and 520-30 shotguns remained in the inventory until the Vietnam War). This early batch of 620s have mostly left over checkered wood if original, they were still using up on hand civilian stocks at this time.
This shotgun is the final evolution of the John Browning designed Model 520 and all of them share the same basic layout and functions (take-down barrel, locking lug, etc) including no disconnector, so yes they're all slam fire.
The Model 520 was designed by JMB in 1903, the patents were approved in 1905 and 1907 and he sold the design to Stevens around 1908, shortly after Joshua Stevens had died. Stevens put it into production around 1909 (it first appears in sales literature in that year). They streamlined the double hump design to create the Model 620 in 1927 and made that gun until 1939 when they modified the design to create the Model 620A. The basic difference is the stock connection (a single long draw bolt vice a cross bolt through receiver and trigger tangs). There is some confusion as Stevens continued to call this model a 620 at times on the guns and in the literature.
The condition limits the collector value in this gun as it stands but it has lots of great shooter value and history.