Black streaks/blotches on 120 negs

Attached is an image taken with a cell phone camera, shot through a magnifier, of a negative from one of the first rolls I shot through my new Bronica S2A. The film is Delta 400.

Can anyone tell me the likely cause of these black streaks/smudges?

I did not get these on a roll of HP5+ I shot through the same camera and processed the same way, but that WAS in a different back. Could this be a result of gunk on the rollers, or light leaks? It doesn't LOOK like light leaks...

Anyway, thanks for any help you can provide.

[edit]
Forgot to mention: These streaks are present most strongly in the first quarter or so of the film, and are more or less completely gone by the last three frames.

Could be dirt/gunk on the rollers indeed. Cleaned off by the first part of the roll?
Doesn't look like light leaks.

Yeah, I was thinking that. I stupidly didn't clean that back out before I used it. That was the back that came on the camera, and I was inordinately pysched to get out and start shooting.

As Q.G. said, it looks like the black inky type gunk that can accumulate on rollers if the back has been stored poorly for some time, or from decayed light seal foam that migrates onto the rollers. So, I suggest not only cleaning, as you now realise, but carefully inspect any seals - though I can't remember if the Bronica S backs have any foam seals - and after wiping the rollers assess if they are detioriating long term, in which case look for a back in better shape: I bought one for a friend a few weeks ago for £30 at a camera fair. Enjoy the camera.

chimneyfinder said:

As Q.G. said, [...]

Click to expand...

Credit to whom it is due: that was the OP's own diagnosis. I just said that i believe he's right.