John it was probably something near the film “gate” of the roll film holder which stopped light from hitting the film at that corner. From my photography background I know that some photographers would file characteristic notches into the film gate of each roll film back they used so that they could identify which one needed repair in the event of problems (light leaks, poor frame spacing, etc).
From my brief bit of internet research, fiducial points (if that is what the markings on these negatives are) seem to be mainly used for mapping applications when aerial cameras are oriented in a vertical position, rather than the oblique views photographed in this instance.