Great question — answering as the UphillTech team:
With the assistance of some vintage North Face backpacks for sale on eBay, I was able to figure it out. The two downward pointing straps thread through the panel on the back of the bag. The tri-glide buckle immediately follows and anchors the straps in place. The remainder of the straps loop around the D rings and double back on themselves. Two more tri-glide buckles—now lost, I crafted replacements out of aluminum—hold them fast. This also holds the stays in position.
Full answer by saltface: https://outdoors.stackexchange.com/a/28087 (CC BY-SA 4.0).
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