Toby: it's not dust on the scanner; I'm scanning 8"x10" prints. I can see it on the neg under a loupe, and on the baseboard while enlarging.
John: I don't think it's something scratching during the development. The reason I say this is the mark is running up and down the "6" part of a 6x7 neg - it's not running down the length of the roll. Does that make sense? I process a lot of 120 film, normally shot on my Rolleiflex TLR. I use Patterson tanks and reels. The mark is on every one of the four rolls I've shot on the camera - the negs were processed at different times.