Using my Canon IV S2 I get a straight vertical shadow on the right of the picture ( It occupies about half an inch down the short side of 6X4 print).I assume it's the cloth shutter edge trailing but how come it's only happening at 1/125th sec and not at slower or faster speeds ? I've put a couple of rolls of film through the camera to test all of settings. The flash photos were fine (thanks)
I had a very similar thing happen 20ish years ago on my old Pentax K1000, the tensioning mechanism on the curtains was damaged and the rear curtain was traveling faster than the front curtain. Not an issue at slow speeds where the front curtain could fully open before the rear curtain started to close, but at high speeds the rear curtain would "catch up" with the front curtain before the front curtain had fully opened.
My situation was that over 1/250 the curtain would catch up about 55% across the length of the frame.
Sadly, I didn't know this and got back from a month-long overseas trip with about 1/3 of my images missing the right half.
Obviously not quite the same thing, but there are a few other mechanical issues that could cause similar situations with out of sync curtains. For example, the curtain "catching" at that speed due to the position of the internals and getting jogged into its final position by the rear curtain closing against it.