Fulltimers your very helpful observation that the two tabs on the back are a switch got me busy with the meter to see what is happening. Indeed the knob for the pot starts with a "click" and that is turning power on to the pot. The "wiper", on a good switch, does have a voltage drop from nearly 4 volts when fan is zero speed, slowly dropping to 0 volts as the fan increases to max speed.
On my problem pot, when it is switched on, the voltage immediately drops to 0 volts and max fan speed is noted.
I did not find a pot that also had this switch so will add a little toggle switch along with a replacement 500k pot, all on order.
Thanks to all who offered help!