You're right - it can't be a light leak. Sometimes it's hard to wrap ones head around the problem!
Your idea about it being a thread or hair was a good one. I put the camera up on a tripod, put the mirror up and put the shutter on bulb. I used a flashlight and looked very carefully but could find nothing.
It's a shame, as the camera itself is in wonderful shape and the lenses are immaculate. It might remain a mystery!