The backplate looks like it was patterned after a standard-prescription M7.
This is pure speculation, but during and especially right after WWII, tiny manufacturing and home-made shops sprang up in Germany, which, lacking inventory and machinery, improvised on the basis of the technology that already existed: M7 backplates, PVC membranes, Neumann capsule head designs.
This could be one of them.
I am most curious to understand how the tension may have been adjusted (I see the extra small slot screws on the diaphragm frame) and why.
