Okay so first off, the drum mag is an attachment I am pretty sure, it doesn't just come standard with it. Because I believe that they could only use the setups that the devs had placed in the game. They couldn't actually change anything. If you look up on google there is multiple types of thompson's which makes me think that this may be varients of the weapon. Perhaps different styles of the weapon? I seen picture that looked to be from Cod WW2 and it looked more like a tommy gun than a standard M1A1.
1. No need to shout
2. It's an Alpha build. Maybe that model of the thompson was finisht and and other not yet so they used it as a place holder. It could be we get different variations.