ok, got it, finally:
‘RTKbase on Futro S900 x86 thin client - #12 by wjr’
The German reference system is called ETRS89/DREF91 (R2016)
‘Satellitenpositionierung’
And it looks like everybody is calibrating their systems to that.
No projection back and forth to WGS84, just bending - or defining, we might say… 