Weird issue with back on RB67

I have a couple of ProSD backs for my RB67 SD. I noticed that with one (I don't know which one as I have mixed them again but I know it is one of the two) I get a weird issue where with most rolls the first frame is really close to the edge. Every time the lab tells me so and in the scans they even include bits of the tape that holds the film to show me how far up it is. The rest of the frames have the correct spacing but obviously all are now shifted forward.

Now, I have loaded film on the RB countless times so know how to align it right with the indicator. I suppose the issue is that once the back is closed and I advance to the first frame the crank slips? But if the crank slipped why would it only slip to the first frame and not the rest, giving me overlapping frames or other issues?

:confused:

Wind the arrows 0.25 of an inch past the datum marker in subdued light & stick Avery label below wind on lever, '+0.25"' as it is easy to swap inserts between clamshell.

Mine are all too large at leader end...

Is it a case of swapping inserts between backs? I didn't know they are sensitive like.

Not with RB67 pro clamshell and inserts according to user manual and my usage.

Download manual for your camera it is free...

An Avery label is cheap ~ all my gear is festooned with labels. eg exposed 120 & 220 film I sometimes damage exposed ends and use Avery to secure as salve.

Carefully count how many full wind strokes are necessary to go from the start mark to the back stopping for the first exposure. Do this for both of your backs. If there is a difference, you may have a problem with the counter.

Also, look carefully at all aspects of the film path in both backs. If you see a difference, your problem could be there.

Thanks Matt, good advice!