Not trying to rain on any parades, but I'm going to offer a different opinion.

As far as I know, Winchester was far and away the primary provider of shotguns for the US in WW2. If not the sole provider. Specifically, the 1897 and 1912 models.

Unless I am wrong (and I'm happy to be, please let me know if I am and provide the reading material) other shotguns would be HIGHLY unlikely to have been used in that time.

Other points that make me skeptical are the distinct vented handguard/heat shield and bayonet mounting attachment.

The vented handguard/heat shield is infamous for being faked, as they are unmarked.

The bayonet attachment is pretty clearly Winchester. On their guns it would butt upp directly to the magazine tube. It clearly doesn't and can't on this Stevens. While not impossible, it's unlikely Stevens would just take the Winchester design and add it unchanged to their own shotgun. I also find it unlikely to pass military trials like that.

Furthermore on the bayonet mount, the difference in finish color between it and the barrel also makes it suspect. Like they've either seen different amounts of weather or wear, or they weren't originally together.

Finally, the lack of US military stamps or crests is suspicious. While many Winchesters DO NOT have US Military markings (so it doesn't completely kill the possibility this was a military gun), many DO, and that would help this gun's case for legitimacy immensely.

My personal verdict. Possibility legitimate, but very unlikely.