It's a RB67 Pro SD
I always pull the darkside out completely, so that wasn't it.
I always use the mirror lock-up.
The problem film was loaded first, a few weeks ago. I just finished it that day then loaded the one that turned out ok.
I got a sick feeling when I read Matt's suggestion that I might have developed the wrong roll, but that wasn't it.
I must have somehow loaded the film wrong. Now I'm thinking I should check all 3 of the loaded magazines.