I had a 50 cron collapsable that was in very good shape (no cleaning marks on front or rear elements), but I didn't find it to be quite sharp enough for my liking. It did have a wonderful level of contrast when shooting it with B&W film. Back when I still had my M8, Leica was telling folks not to use the collapsable 50 cron on the M8 cameras because of a fear of the back of the lens contacting the sensor glass.
I eventually sold it and purchased a 50 Cron current version, before they were doing six bit coding, so probably from the late 1990's. I LOVE the new lens and don't regret selling the collapsable one for this one. Contrast is higher, but still very beautiful with B&W and not at all bad with color, and the little thing is sharp as a tack.