You say you are developing these for your customer. I would look to what the customer has done.
I collect old cameras and these images look very much like what I get with the first film through a new acquisition of an old camera. When you wind-on the film, a very small static charge is formed...
All sepia prints were made from black and white negatives. There is no such thing as a sepia negative. The date is immaterial - black and white prints were being tinted sepia a very long time before 1930.
I would think that you have a slight light leak in the foam light seal at the bottom of the Yashica. When you wound the part used film through the Yashica, you will have done so very quickly, I suspect, so the fogging by the light leak will have been too small to notice. When actually taking...
In a waist-level camera, the viewfinder image should be the right way up but mirrored left to right. Composing is easy once you get used to it as is keeping verticals vertical. I find waist-level finders are a good aid to composition (not because of the reversing thing) as a direct result of...
I don't think any German camera makers used foam light seals. Germans like engineering. Certainly, none of my Zeiss Ikon, Praktica, Balda or Voigtlander cameras have any foam light seals.
The lines are on both sides of the negative, though.
Strikes me as more likely to be diffusion of unused developer from the area between the images causing greater development just inside the image.
Were you wanting to share or did you want to tell? All amateurs know things you don't and I find listening pays dividends – the more you listen to others, the more others will listen to you.
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