How to calibrate the vertical sensitivity of an old analog scope (Hameg HM203-6)?
As suggested, I just tried it and calibrated it myself. The procedure being as follows:


Disconnect the power cable, and remove the scope cover;
Place the scope vertically, so the VR703 pot can be reached (and the front panel still reachable);
Reconnect the power cable, turn the scope on, and supply the reliable 5 volts (in my case it was through a digital Lab PSU, so I also rechecked with 1 volts, 2 volts, etc.), and adjust the pot to get the expected value;
Turn off, reassemble, done.


Nothing scary, the scarier part was the amount of spider webbing inside :)