[Jeffhs]

I'm surprised this unit has a volume control; after all, it was made to work with the audio section of a radio, which of course has its own audio amplifier and volume adjustment -- therefore, the phonograph attachment did not need a volume control, as the output cable was connected directly to the cartridge and terminated at the other end in an RCA pin plug. (RCA Victor's 1950s-vintage 45-EY-3 45-rpm changer, also for use with radios and TVs having phono inputs, was wired the same way.)

My best guess is that the hole on the motor board of your R-93-B 78 phono is probably for an on-off switch, maybe rotary but it could have been (and could be replaced today by) a push button; in a pinch, even a toggle switch would probably work. For testing purposes, you could even solder together the two wires from the switch and use a power strip to switch the unit, as well as the radio with which it is used, on and off.