Thanks Bertil. That very sharp contrast fall-off on a couple of the frames also is bugging me: really it must be some issue on top of the developing issue. I've changed seals in my backs and I'm going to incorporate yours and Sander's development suggestions and we'll see if I get this again. It's altogether possible I stupidly half=pulled the darkslide with the back open or something like that! I do dumb things like that all the time.
This have been very helpful to me and will avoid messing up future rolls. Thanks for checking back in Bertil and for the pm!