I picked up about 50 old wattmeters at a junk yard about 6 years ago($1 each! and all the different amps, volts,and phase styles I could ever need). The local power company was switching to all remote read meters and was just scrapping the old ones. I have at least 15 different manufatures in the lot. Every standard 120/240 200 amp of each manufaturer I have played with runs that black bar on the spinner past at a different rate. But they all read the correct KWH at the dials and numbers.
The one remote wattmeter reading device I looked at used an IR LED and sensor and just watched for when the black bar went by. No forward or reverse sensing that I was aware of. I did not get a look at the instructions though, so I do not know if there was calibration settings to make the bar count acurate or not.