John E. wrote in
obal.net:

Axial electrolytic
approx 0.5 inch (12 mm) diam, 1.0 inch (25 mm) length

Markings:
HGC
CP-C
2J104K
123

Google tells me it's a 0.1uF, but I can't find a reference for the "2J"
voltage rating. And, any idea what the other markings are?

There are no manufacturer markings (logo, etc.), just plain gray shrink
tubing with the above alphanumeric markings.

This is used in a single-phase 240VAC low current box fan speed
controller (1 TRIAC is the main current controller).

Any help would be greatly appreciated.

Thanks,

That's a damn small capacitance for that size, though I agree with it, so it
implies a high voltage to match what you say. I first thought of 123 as
meaning 12,000 millivolts, after the convention of 104 for capacitance, but I
never heard of anyone actually doing it this way, and it's definitely not
12V.

Is the writing clear? Any chance that 123 is actually something else?