Up to 1957 the orifice was in the fitting that screwed into the block at the rear, under the side cover, that the rocker supply tube attached to.
Starting in 1958 Chev propped the supply tube under the side cover and changed to a cast in supply galley. On the 1958-63 engine, if you look under the side cover, in the center top, you will see a 1/8" pipe plug. The orifice is behind that plug.
Paul, I would suggest pulling a rocker from the shaft and see where the groove inside it is located.
