Tim,
Since [MENTION=114373]northbend[/MENTION] now owns the bag, he’ll have to show assembled details.

But the key parts to the “quick release” that aren’t too clear are small metal (stainless steel, as I recall) spring hooks like these:

Mini Hanging Buckle, Uning Pack of 20Pcs Metal Spring Backpack Clasps Climbing Carabiners Hook EDC Keychain Fishing Camping Bottle Hooks Paracord Tactical Survival Gear (Silver) https://www.amazon.com/dp/B06XBBN5KF..._4pioAb33CS6K8

Sorry I don’t have any to photograph, but the metal rod inside the bag passes through the circular closed loop on these hooks, while the hook and it’s spring closure extend through the holes that originally allowed the leather strap to pass into the bag. I cut squares of rubber sheet with a small center slot that the hooks pass through to prevent water entry through those holes.

These hooks are just long enough to clip through the metal bag loops on Brooks or similar saddles.