While at Ely NV, I got a definitive answer about the mystery valve ahead of the engineer on the Alcos.
It is the triple valve. One of the RS-3s at Ely, has it along the short hood on the left side, when viewed as long hood forward.
When I mentioned the RS-1's valve, it was confirmed that that original location is along the long hood on the engineer's side.
Old farts are so fun to talk too...