Can anyone tell me how the frame numbers from the back of the paper on a roll film end up on a negative? They have the exact shape of the red safety window so obviously the film was exposed through a red safety window AND the backing paper. I just don't understand how this can happen. I have shot other films with the same camera that showed no problem so it can't be the camera.
The camera is a Zeiss Ikon Nettar and the film is Macophot UP 100 plus (epired 2008) rollfilm developed in rodinal 1:100 for 47 minutes.