A wide array of shotguns were used for aerial gunners training and gaurd duty during WW II as dave mentioned. The most common shotguns found in combat though were the Winchester Model 12 and the Winchester Model 97 with 20" barrel and the heat shield/bayonet lug attached.
I recently had a Remington 10A with Ordinance stamps on it and a 20" barrel. A factory gaurd gun? A cutdown by troop sin the field? A post war cutdown? Hard to say but I'd bet on the first or the last scenario if I had to guess. Although the condition was poor enough that it could have seen combat
