Shot two test rolls in a new to me Canon IIb. All the frames seem to have measles. The lens (Serenar 50mm 3.5) is clean, with only normal internal dust, and that's checking with a flashlight, so the only other thing I can think of is pinholes/deterioration of the (1st) shutter curtain. Because of the Barnack design, I can't just hold a light on the other side of the shutter, so I turn to your expertise.
Some examples of the frames (every frame has the same pattern, and some are worse then others):
Some examples of the frames (every frame has the same pattern, and some are worse then others):