Yes I noticed, we tried to discourage that because;
1 It made the whole thing much heavier.
2 If you lost a mag, you lost two.
3 It made the prone profile of the shooter much higher.
4 While prone you were subjecting the open mag to whatever was beneath you.
Reference the stock, I've shot my share of Thompson and I don't mind the stock. It gives you a point of ballance for point shooting. The thing about them is they tend to rattle when a bit worn. I wonder if that was what this thing you mention is about. It's hard to sneak when your Thompson's giving you away. Remember the M1had a fixed stock so that ended the rattle.