Every now and then an old fart like me can get in a discussion cause I think I still remember. Anyways, we checked main open, checked line voltage, 120-120-240, checked both line to load sides for 0, put temp shorting bar across one side, checked other line to load side for 0. Removed bar, rechecked meter on line side to make sure it was still working, and then installed meter. No digital meters in my day on truck.
"Growing old is mandatory, growing up is optional."