Hi Paul,
I've seen this on a few cams and thought that it could be common. I posted about it here a while ago, but it seems that few have suffered this problem. It's not directly caused by bad curtain tension.
As you have seen, the curtains start off barely overlapped and increase their overlap as they are wound on. This is normal.
New curtains fixed in very slightly the wrong place can make this problem much worse.
Sometimes the metal laths on the curtains have a curve which does not match its partner or the light baffles at the top and bottom of the curtain track has a clearance which is far too large.
Some of the things that I tried on mine were to wind to half cocked and to check that the laths have a reasonable seal. If not, gently push the pair from the front then the back of the gate to encourage a matching curve.
The light shields play a part here, if the overlapping laths have too much clearance they can part, so move them to give near contact on the overlapping laths (at a pinch, you might do this by bending the shields, always slightly and always gently, to achieve a suitable gap of say 0 to .1mm max) if too tight, you will feel it and hear it scraping the shield when you wind on).
Some of those marks could be down to other things, so, personally, I would use it as it is (covering the lens when winding on) to be sure that it is only this which is the source of the leak.
I have one FED which does this infrequently, so I've learned to live with it (or is that round it?).
Good luck with the replacement..
Dave
