One of the supermeters was reading wrong DC voltage, but all other functions were correct. It seems it somehow lost voltage calibration, perhaps someone accidentally set it to the calibration mode. For future reference, here is what seems to be the calibration procedure, from trial and error. The meter is using MAX133 ADC.
Switch internal jumper from middle to lower position (closer to the fuses). Turn on the meter to the desired function. The lower display will show the required level for calibration in ADC counts, like 39000. Not sure if it is possible to change the calibration level, the default for voltage function is 0.39 V, 3.9V, 39 V, 390V and 600V (6000 counts) on the highest range. Switch the range and apply proper voltage. Then press Max/Min, wait for a few seconds and then press it again for longer to get out from the REC mode. Then switch to a different voltage range using the range button and repeat. The annoying part is that one has to calibrate all voltage ranges, otherwise uncalibrated ones will be corrupted. When done, press backlit button, the display will turn off. Turn off the meter and reset the internal jumper to the middle position. Check that all ranges got calibrated. For some reason it took me a few trials to get them all correct.