I have found the trace rotation pot on the Z-axis board.
Possibly the 7613 found it's successor in the 7623 because of the difficult
access of the relevant pot.
There's no cover hole to access T-rot.
But, it is labeled indeed... , P1170... like all the pot's on the Z-axis
board are numbered.

Anyway, traces are fine now, flat like the Dutch horizon :-)

Thanks everybody for helping out!

Rgds,
Jan.