This is the first piece in a larger lot that I decided to tackle. It’s the simple cousin of the Heathkit IT-18, which also measured DC voltages & resistance.
I beat my head against this for far too long until I realized that the calibration pot would sway wildly out of spec depending on the position of the contact – from 40Ω to hundreds of ohms. I replaced it with a 50Ω pot and it was able to be calibrated to mostly within spec. I’ve never found these early, cheap VOMs to be particularly accurate, but they’re fine for basic work.