Like Rudy in "Funhouse" would say: Oh boy, oh boy, oh boy......
I just decided to take off my wooden backbox cover (I finally had a hunch) - only to find that the DMDs were there all the time, hidden and squeezed in the upper left screen corner, covered by the wood cover!
OMG.... But how did it get to the upper left corner of the monitor?! Anyway, with the coordinates from Screenresidentifier I was able to reposition it in PBX Settings.
D'OH!!!
Thanks a lot for your time amd patience here.