The TU-75B is a Variable Line (0-150 V) Power Source Test Unit combined with a current limiter primarily used inside of the factory at Test-Final for initial test and eventually to calibrate instruments that came off the assembly floor.
It's a Variac set up for 0-150 V and the E Out/Line switch is for the AC voltmeter.
The 1-2-3 switches provided the choice of one of the two internal incandescent lamps for series resistance when the scope was turned on for the first time to prevent damage from shorts that weren't discovered during a visual inspection. Seems like there was something like a 150 W and a 300 W lamp choice. When all was deemed to be OK then DIRECT was chosen to eliminate the series resistance.
The scope was then given a rough check for proper operation and put in a special room to cycle until the next day where it would be calibrated, sent to QC and then shipped. The DIRECT position was used in the cal procedure to check the power supply regulation from low to high line.
The TU-75B is similar to the 75A.