i've got a few of the USGI buttpacks unsub and they look very similar to yours.i also have a nice blackhawk one and i can fit 2 USGI gore-tex suits in it or 2 USGI gore-tex bivvy bags in it.

i want one of the newer ones some companies are offering in the 1000-D CORDURA and with MOLLE webbing on it.they work great with a shoulder strap