This is a Woodland Camo ELCS (Equipment Load Carrying System) Large General Pouch. This pouch is a great way to carry bulkier or larger items you need on your kit or bags. The BDU pouch has a full length zipper and is MOLLE compatible.
The ELCS is the predecessor to the SFLCS kit and gear we have in khaki from Eagle Industries! The US military primarily issued these to Special Forces.