Is there an adjustment pot on the MFJ259b for the freqency counter and
if so, what number is it on the board?

Gary-

It looks like the counter module is purchased as a unit, probably from
Optoelectronics. Therefore, MFJ may not know anything about it they don't need
to know.

I've looked for anything that might calibrate the frequency, but couldn't find
it. Perhaps further disassembly would have revealed a hidden trimmer
capacitor. I doubt it would be a pot, but you never know.

How far off is yours? Mine is close enough for its purpose of checking
antennas, but I wouldn't adjust a transmitter's frequency with it.

73, Fred, K4DII