I got hold of 3 plate holders for the 9x12 cm plate camera which I still don't have identified properly. Short: compur shutter ZDM/IBT with "DF" marked on the front of the shutter time wheel / rodenstock f3.5-13,5cm lens #321392. I will put up some pics to question that issue in another thread.
Holders are 101x150mm x 4mm thick, edges 2mm 3 way around.

Novice 9x12 question: do I need to replace the green glass and put in a 9x12cm filmsheet to use the holder or do I put the filmsheet on top of the glass?

tnx for your answer!