Howdy folks

Took a walk with my 500C/M today and an old C-12 back (that I posted about a few days ago.) Out of 12 shots, only 3 had light leaks. I remember when shooting this that the sun was to my back, and I think this is light coming under the door covering the peephole on the back. My friend thinks the light is coming in from the dark slide slot.

Anyone else want to guess? Or know for sure?

Edit: All three light leaks are the same shape, in the same area. This is the longest.