@Steve and Helmut: thanks for the hints.
I started to doubt that I can see a change of the TX freq (while changing the hidden adjust hex value) after I had a look in the service manual - the "frequency scheme" picture there.
It could be the changing of the voltage (while changing the maintenance REF OSC hex value) changes the oscillator's frequency actually, but the TX frequency stays unaffected because the change "somehow cancels out" within the frequency scheme. It is not easy to formalize the change from that picture, though.
Hopefully, the setting the master frequency as described in the service manual will help with the aged crystal..