From the colour it looks like the leak is from behind that is through the film base e.g.
- through the door seals
- or into the sprocket region before the film is reversed
You could black electrician tape over
- the back plate top and sides
- the baseplate join with the body, all the way round
- the baseplate latch
- battery cover or self timer
- rewind control
- exposure counter
- speed dial, use lens for control
- shoe
The lens and mount should be ok and the viewfinder, lever wind and rewind are too difficult.
This is not silly if it were the baseplate you could replace.
Lastly two things
- do you have an ERC if so use it, and push a black nylon sock or scarf around the top of the case at the rear, and down the sides at the front.
- do the seals on the camera for the door seem ok, I don't know what seals a M6 should have?
Noel