Quote Originally Posted by Tom Stanworth
If it had fogged it would have looked like any fogged frame (darkslide removed off camer etc). This does not look like this. As I say, teh undeveloped leader from the same film does not clear either. I bet something strange happed during manufacture...This is unlike something I have ever seen...the base still has a that sort of unprocessed look to it...I may even try to print one, though the exposure would probably be a minute or two....
The "reversal exposure" would have occurred DURING processing... somewhere around the mid-point (??) of development.

Hmm... A thought ... did you use a developing tank? If it was a JOBO (??? I can't remember - did you say it was a 1500 series?), was the conical light trap in the cap (fits into the central "spindle") in place? That might explain light entering the tank during filling and emptying - if not during the development itself.