Part of the standard issue US Molle II system: the waist pack. Also more colloquially known as the butt pack.
These are in ACU/UCP (that grey digital camo that blends in with a gravel pit and grandma's couch)
They have a waist strap with buckle, along with a zipper for the main compartment. Additionally, there are two molle straps to attaching this to a FLC or rucksack.
These will be from a variety of makers, but all are genuine US Government issue and in excellent shape.