Given the incredibly long market life of these C3s, I suppose there may be internal changes along the way, but . . .
I have one I bought new around the end of 1957 (which just successfully worked for Argust 6th) and I just got it out to take a look. All of the following think-out-loud comments are based on my one sole example:
a) There are NO light seals in mine. It has molded-in ribs and shapes that create a sort of labyrinth seal around the back.
b) The hinge end is behind the film cassette, so I doubt that would be it, but are all the screws in the hinge?
c) The viewfinder and rangefinder stuff appear totally isolated from the film chamber.
d) I think (applied optics here) the spot is toward the lower left of the exposed frame as viewed from the rear of the camera.
e) So I tried to assume the spotting might be occurring on the previous or next frame, not the one actually lined up for the exposure.
f) If the frame is being hit prior to the exposure position, the light would have to hit between the right side of the masking frame (viewed from rear) and the sprocket shaft, a little below center. In the body, I only see one little plugged hole that's even a candidate, and it would be covered by the front plate. The other possibility would be the rivet that holds the pressure plate to the back at that area -- is it intact and tight? (Even that normally has the leatherette covering over the outside.) You might skip exposing a frame or two either side of a properly exposed frame, winding quickly through the unused frames (or in the dark). That might check for shutter leaks vs off-frame. Ideally, don't wind to, or take an exposure on, the last frame. If light is hitting next to the cassette, the last frame will have the spot anyway. If not, that end is clean.
g) I really want to suspect something over near the catch end, but approximating where your spot would sit once the film is wound to the next frame, I think it would be past the latch section. I do note however, there are two rivets that hold the catch onto the body -- if the lower one of those were corroded away, that could be a candidate. There are three or four plugged holes toward the front of that film chamber, but again, all of those are covered by the front plate. (And possibly if whatever is in them falls out, the shutter stops working!)
h) My other reservation about the take-up spool side is that the diameter would grow as you work your way through the roll and I would expect the spot to slowly move a bit laterally with each frame. From your three shots, that might be happening but two look pretty similar.
Well, good luck -- that ends my forensic engineering for the night!
DaveT
