edge shadow on negative 6x7

I just developed a couple of 6x7 negatives from a mamiya rb67 camera and
some of the frames have a very light shadow on the edge of the frame. There
is no detail in the shadow just a very light shadow along the left and right
sides of the image. It is not there on all frames. When comparing the
affected frame with that of a good one the image area is equal except the
slight increase that the shadow creates.

Any ideas as to why this may be happening. I assume it is no real big deal
as it does not seem to affect the image within the frame but I am curious as
to why this happens.

Don