This is frustrating.
I just tested new-to-me Mamiya RB-SD 645 back in the field. As you can see, there is a light leak on bottom of the frame. An odd thing is, this is the only place it appears - overlapping frame 1 and 2 and in between. It happened no where else.
This is a back that came from a well-known source and was in great condition. I don't think it was ever used....
Film was Tri-X.
On the image, you are looking at the film from back side as you can tell from the frame counter number. It is at the bottom of the negative.
Does anyone recognize this pattern? I don't see any damage of any kind on this back. It really looks brand new.