Until later in the M6 Classic production, Leica used strips of flocking in front and behind the shutter curtains (down in the crevice below the film area) as light seals. These it seems can sometimes get worn out and let light leak through. The current design is a v-folded piece of black mylar which is cheaper to make and install but more effective. DAG I know will replace the flocking seals with the latest kind, I don't know about other repair people who may reject the new seals as "cheapenings". My older Wwetzlar M6 has the flocking seals, if it ever needs an overhaul, even if the light doesn't leak, I'm going to have DAG put the new type in. That said neither my M4 nor my M3 leak light and they've got the original seals.