Hi everybody. I bought this backpack in the late 80's from a military surplus store. I found that the store never included the strap that holds the backpack to your waist (not the kidney pad). I need to get this, but where?

I have the two padded shoulder straps that attach to the top of the metal frame but each strap ends with a closed black nylon loop, so I can't figure out how to attach these two closed loops/straps to the two horizontal slots in the bottom of the aluminum frame. Can anybody assist me with this?