I'm very grateful for all the comments received, both here and in private conversation.
Some context:
I've been developing roll and sheet film for several years, 120 film in various sizes (6x4.5, 6x6, 6x9, even 6x17). Which is merely to say I've made more than my fair share of mistakes. But it was only when I started using the RB67 in January of this year that the mysterious mark appeared. Hence my belief that the camera itself was at fault.
A belief I no longer hold, as it now seems most likely that in spooling the film I've somehow (and all too consistently) encouraged the leading edge to interfere with the development of the frame opposite it on the reel. (Looking at the roll as it hangs to dry, I can see quite clearly how winding brings these two together.) So it's not the camera or the film back or the reel or the tank ... it's me.
Back to basics:
When, in the dark bag, I remove the paper backing from the film, I hold the roll in my left hand and unwind the paper with it until the film emerges, which I then collect in my right hand until both parts have unrolled, at which point I separate them with a gentle twist. (BTW: Is the emulsion on the inside or the outside of the curl? I've always assumed the inside. Now I'm not sure.)
The next two steps I've begun to question:
1) I then cut off the film edge with the tape on it, making a new edge more or less even. Should I not do this but simply peel off the tape? (I don't know why I do this; I suspect it's to prevent tape residue from contaminating the film.)
2) In guiding the film edge onto the reel I usually pull it past the ball bearings. Should I not do this but expect the bearings to do the pulling themselves?