Part of the standard issue US Molle II system: the waist pack. Also more colloquially known as the butt pack.
These are in OCP or Multicam, Almost ALL CIF facilities will take either or in place of the other.
Associated NSN(S):
NSN: 8465-01-580-0481
NSN: 8465-01-580-1300
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.