I have only had this probelm with Efke.

I took the backing paper and reattched the negatives to it, and iscovered that the images of the numbers appered where the numbers actually touched the emulsion, in other words they were offset to where light comming through the back of the camera could cause them to appear.

Avoiding this sort of trabsfer is the reason that has been given for the Light coloured print on Ilford roll film.