D-BUTT PACK GEN. PUR.
Eagle Industries ButtPack/General Purpose pouch in AOR1. There is one mark on the corner of lid. Appears to be from handling.
Tons of pockets allows you to carry a wide variety of items. It can expand to carry larger items or cinch down to get out of the way. You could even stuff a hydration bladder in there and carry it on your upper back.