Those photos show an M23 subsystem ammo can utilised as a backpack. The M23 was probably still the XM23 back then, but was the mount used for the Iroquois door guns. The ammo can held 600rnds from memory (it's been a few years since I loaded them). There were three or four separate compartments in the can, and you started at the far end, filling each section in turn with your belt, feeding it into the next section as the previous one filled up. The link chute then clipped onto a fitting on the can and then onto the gun at the feed tray.