Thanks for the answers everyone.
Could it be light leakage from the viewfinder, or those little deals that screw in to cover the RF holes? Either from inside it, or out? It appears the light leak is from that area.
It could and it was my first idea but the fact that I can see sprockets inprinted on the negative means for me that there are multiple layers where the light leak touch the film:
I've got the same leak in my iiiG although not quite as pronounced as this. I havent had a chance to properly investigate but my initial investigation showed a tiny piece of vulcanite missing near the slow speed dial. When exposed to direct light it casted a little light into the take up spool region of the camera, which matches the marks seen on the film. i.e the spool clip. What is the condition of your vulcanite? Maybe open up the bottom plate and shine a torch around the camera to see if you can localise the region which is causing the issue. Would love to see where yours is as it gives me somewhere else to check on mine
I tried again to do it insisting on the low speed dial but haven't succeed to see anything inside. Even tried to move thing and press on them but nothing conclusive... Sometime it's bit hard because I think I see some light but trying to localize it I find in comes from the open plate...
Vulcanite is not in great shape, has small cracks and crumble a little but I wasn't thinking to replace it because it was only aesthetic for me and only small things. I bought this IIIg to use it and not be afraid to scratch it.
On my IIIc I replaced the vulcanite by a leatherette because giant pieces were ungluing and I was afraid that the vulcanite would stay in my hand while the camera kiss the ground. But never had a light leak issue.
Here are the defect I can see on the vulcanite:
My next recourse was to drop it to the well known Parisian shop specialized in old Leica for them to try to fix it (and cla while at it but I don't think it needs it, it's already a lot smoother than my IIIc) but if the fix is to change the vulcanite I can do it myself and order a leatherette from Japan...
Note that on the 3 films I shot with it during my travel the frequency and quantity of appearance of the light leak varies a lot so something must trigger it. Maybe an angle to the sun or pressing on something...
Of course on the test film I did before it did not happened at all...