The calibration instructions call for two "calibration units" Tektronix part numbers 067-672-00(vertical) and 067-673-00(horizontal), which for sure are highly unlikely to be available anywhere 30 years after the fact.
From the instructions, it is my GUESS that the horizontal unit routes the input from its front panel to direct to the main X amp input, and MAY also generate an unblanking signal (based maybe on its input).
Similarly my guess for the vertical cal unit suggests that when the "BAL" function is selected then the inputs to the Y amp are referenced to ground via a resistor network or possibly just connected together (possibly via a lowish value resistor) and when "CAL" is selected the front panel input is fed directly to the Y amp input (pins 5 and 13 on the output pcb connector).
That's all on the assumption that these units are pretty much passive - all bets are off if they are active things.
Does anyone actually know what they do? Or even have these plug-ins or the schematics for them?