I am pretty sure that the big valve head was introduced for the 1960 season therefore was fitted to all Rockets. I am also sure that the inlet valve only was slightly enlarged and the exhaust valve stayed the same dimensions as previously.
As far as I know, the Flash valves never altered in size.
Trev.
P.S. The heads can be identified by the number cast into the bottom of the inlet manifold.
67-1126 = Road Rocket / Super Rocket 1956 - 59
67-1549 = Super Rocket, Big Valve head from 1960
67-1571 = Late Super Rocket and RGS. the "49" was ground out and "71" stamped in it's place.
Not sure of the difference between the last two, the valves are the same, but I have seen reference somewhere to the inlet ports being modified, probably for 1962.