the PVC KK50 was NWDR broadcast's idea of a capsule, used in '51-'54, together with MSC2 tube. Neumann then designed the metal diaphragm version, the KK53, for KM53 and M50. PVC just isn't suitable for high mechanical tension.
The KK50 capsule looks like KK53, only with gold-sputtered PVC diaphragm.
Most very early M50s were later reconfigured, with new capsule and/or AC701 tube and/or RF-protected connector version (a.k.a. "M250"). So, a surviving M50s with PVC KK50 will be quite rare, but quite probably it will need a new capsule, to work properly.
Best regards, Martin Schneider / Neumann Mic. Development