This is what I came here to say! I know size is hard to grasp on the internet through pictures but, all you guys suggesting modified pistol belts don't get the size difference between a real (or even a replica) military pistol belt and whatever this thing is.
I chased down the original source, a listing at heritage auctions for a Rambo 4 knife. In the first picture and some of the others in the auction, you can almost see what looks like a tag sewn into the back of the buckle section.
It's definitely an off the shelf piece.