Hello
Carl Sievers (Heiligenhaus, near Düsseldorf) can finally be the answer.
Some papers from the company can be found here :
https://www.zeiss-ikon-schlossmuseum...us/CHS%2C4.jpg
The code for Carl Sievers (Fertigungszeichen) was ojh. Produced spring loaded access hatches, probably for a lot German planes.
Best regards
J Schreiber