Because of this thread I dug out my Furman PL-PLUS E and connected it into 230VAC - now it reads 260V
Then I connected it into Variac and adjusted voltage so the reading was 240 - measured voltage was 212
So I opened it - and there is a flimsy trim-pot - now reading is correct. Whether the setting stays put while in transit remains to be seen. Should be multiple turn trim to be any good. Or perhaps I should replace it with two fixed resistors.
My favorite Stupid Furman Trick is that when the dimmer pot nut gets loose, the whole board rotates on the pot and eventually shorts 12v to chassis, drawing enough current to trip the GFCI on your FOH circuit.