We don’t know which were the thoughts in Scubapro designer’s mind in 1970 while designing the valves in the above picture. One double-tank manifold with only one int connection was a standard thing for that time and the same was the mechanical reserve with manual control. However, a double-tank system with two reserves with only one valve was never seen!
Which was its function? How should one proceed? We know that, to open such a reserve, diver had to pull the stick downwards by the handle comfortably positioned in the lower part, next to the bottle. In this way, the diver was turning the latch placed next to the valves. I believe that, also in this unit with two reserves, it was certainly foreseen to operate one reserve as soon as breathing was becoming hard, but what about the second one? It was a spare one, if the first one was stuck? Or it had the function as reserve spare in case the air freed by the first one was not enough for the diver to reach the surface? Difficult to imagine, considering that in the most probable case of endless disuse of the second reserve, diver turned down to those 30 bars of air blocked in one of the two bottles in every dive! Surely, only a crisis of extreme pessimism may have suggested to the designer such a technical solution!