i'm not sure which was the standard, but i found this list of ww2 shotguns for you.
Ithaca Model 37 Trench/Riot guns
Remington Model 11 Riot gun
Remington Model 31 Riot gun
Stevens Model 520-30 Trench/Riot guns
Stevens Model 620A Trench/Riot guns
Savage Model 720 Riot gun
Winchester Model 97 Trench/Riot guns
Winchester Model 12 Trench/Riot guns