I'm on seventh re-loadable cassette and so far no light leaks with M4-2.
I was advised by local RFF member not to advance film in advance, because same looking leak appears to be this way. Film advanced and if shutter is not released for some time - light leak.
I remember GW and HCB videos - they never advanced and leave.
But, I'm just not good enough. Every time I get rare chance to do real street photography, I'm loosing shots by advancing in last moment. I could do tab focus and aperture, but if I have to advance film as well - it is often too late.
With Summarit-M 35 2.5 and its hood I found my own way. If I have film advanced, but shutter not released, I keep on walking with my palm as lens cap. It is great hood for it, no holes in it and this particular lens aperture ring was designed and build by those who understand what is classic street photography with Leica. It is ring which I'm able to operate with single finger and without looking at it. I keep it at f8 - two clicks one way f11 for bright side, two clicks another way f5.6 for shadows. I do it with single finger, while lens is still covered by same hand and by next second I'm bringing camera to my face while setting focus by position of focus tab.
It is about three seconds. So, I should learn how to advance and focus by same time...
Anyway, here is photo taken with M4-2 and Ultron 28 1.8.
After shoveling of snow at long driveway.
It is great lens ... by all means. Too great in size. Just as Elmarit-M III 28 2.8. It was awesome on negatives, but too much of the VF blocking. I like to use entire M4-2 VF as 28mm frame. It works, but lens blocks one quarter of it...
More and more I think about first version of Elmarit-M 28 2.8 ASPH. I like how small it is, how no distortions it is. Same focus tab as on Summarit-M 35 2.5... It could replace my Cron 50 V4 and this Ultron 28mm lens.