Just a curious note but I have about five assorted old style plastic back plates. These were made in the 70s and 80s. They ALL have a molded in tunnel that has a built in stop at the top of the tunnel. The back plates were intended to be installed so that the stop rests agaisnt the shoulder of the tank. When installed as intended by the manufacturer the tanks sit low with the valve well down between the shoulder blades. Had the tunnel been made full length then the tanks could be set at any level but apparently they felt it important to position the tanks low. Curiously the position that results is the one recommended in scuba texts of that era, low. I have several older plates from the 60s, a Voit Snug Pack etc, it does not have a stop and the tunnel allows the tank to be positioned high or low or anywhere. This leads me to believe that at some point it was decided that constructing back plates to properly position the tank was important enough to build in a stop. N