Thank you for the information about Kodak 35. It was produced about the same time the pictures were taken. I should look at Kodak 127 type cameras of 1930s. It is also possible that the camera was British. The supplies of German cameras to the UK stopped about 1939-ish. We are looking for a simple folding/strut camera that has issues with locking the lens board parallel to the film plane.
I've never seen these notches mentioned in any camera manuals or processing handbooks. Maybe the manufacturers did not want to confuse amateur photographers with additional information which would be of no value for them since their negatives were developed by the local pharmacy.
Before this thread of yours I was not even aware of theses notches, though having at least one, maybe two models with such. I guess I am not the only one.