Not at all familiar with the old 15" sets but.....
I assume its a voltage divider. What is the typical voltages on it ?
Lots of 70's to mid 80's SS sets used very high ohm focus dividers & bleeders.
Also focus pots that went to at least 15 meg. Should be able to put something
together that will work & not look messy.
73 Zeno
LFOD !