Yes... another magazine...
... on the wrong side of the gun...
... blocking the shell EJECTION port (ejection, not intake)....
.... too small to actually hold any rounds in it....
So yes, "maybe it's another magazine"....
The only alternative I can think of is that there were 6 rounds in the "chute", but the gun didn't start right there. The tube was hollowed out, and the gun sat in the very center of it. You can fit 1, maybe 2 more between where the chute ends and the gun begins, plus another in the barrel ready to fire... That would be 6 + 3 maybe, 9 per gun. This is backed up by the ammo loading holding a stick of 6 in his hand, but you being able to see rounds inside the chute opening already...
It's all a matter of how they were packed and fed into the gun, I guess. The MG/FF ammo drum carried 60 rounds, but later in the war was repacked to hold up to 90 rounds in the same space. It's possible there were 12 rounds per gun, just not in a single "stick" like you might think.
