The first of the military riot guns I know of made by Stevens/Savage was the model 520 pump. This was not a 1897 look alike as it was hammerless in style , with a "hump" back simular to the Browning A5 reciever. It was made off John Browning patents, and was post WWI in production.
The only other exposed hammer pump I know of was the Marlin , which I don't believe was ever used by the gov. I also am not aware of any other manufacturer that was contracted to build the Winchester 1897 type exposed hammer gun. So I am at a loss as to explaining your sighting. Any more details might help . For example the Stevens model 520 and 620 were made as trench guns with barrel shrouds and bayonet lugs, but were WWII usage firearms.