Almost deleted post after finding the screw size in the Motorola factory service manual, but thought that the info might help others...
For the X19 transistor radio volume control knob, a 1-72 x 5/32" screw is specified. Since it's not metric, maybe this isn't the standard size used in other Japanese made controls ???
I found over the years that some Japanese machine tools use BSP (British Standard Pipe) hydraulic and pneumatic fittings, not metric, so maybe controls on old radios were non-metric also.