I used to have a link to a page that went to great technical length explaining how this is a light glare on the lense which creates a reflection of the aperture back onto the inside-center of the lense. You can clearly see the light glare on the upper left and the aperture reflection in the center which is octagonal like the aperture.....
Also. These cameras have to be wound up for the next exposure to be taken. From what I understand, the camera was found in the wound position and ready for the next shot. When the lab has to open the camera to retrieve the film for processing, the lab had to physically snap that wound shot before being able to open it up.
The very fact this is the last exposure in the series on the negatives lends credence to the above scenario being the case.