From photos I've seen in books, I believe the air reservoirs were added by the NYC sometime after the RS-3s had been in service a few years. I've seen photos of the same locomotive without the reservoirs on the hood, and later with the reservoirs there. Also, it appears, in my opinion, that NYC planned to increase the size of either the fuel or water tank after they moved the air reservoirs to the hood. Some locomotives have the area where the air reservoir previously was filled out to the edge of the frame. Others do not have this extension. Maybe, again in my opinion, the NYC modified quite a few by moving the air reservoirs to the hood, and extended the underframe fuel or water tanks, but changed their minds about the fuel or water tank extensions, because most of the photos of RS-3s with the air reservoir moved to the hood do not have the fuel or water tank extension. Carl
Carl