thanks for the quick replies!
The interior is still all black. It has recently (and for 1 and a half years) performed without a problem. The back seems to be fine and fits well.
Three rolls have been developed that exhibit the issue, two at one and one at the other. Both reputable and I have developed a lot with each of them. Plus, the spot, when it appears, is always in the same spot, so it would have to be something really odd to do it every so often in a perfect 6cm spot.
It also appears the back plate spring is still okay, so it should be putting pressure on the film and I believe it adds to the light-proofing of the door.
Again, thanks for the thoughts. I'm baffled.