I'm not 100% sure, but it's my impression that the M 934 C was always made with a Mylar diaphragm.
--About a dozen years ago I bought a pair of M 221 B microphones with M 934 B two-pattern capsules from a seller on eBay. I sent them to the factory for checkout and repair, and was told that one of the capsules was water-damaged and had received a new diaphragm. (The eBay seller had of course claimed that the capsules were in "perfect" condition ...)
I then used the capsules for live stereo recording as a pair of closely-spaced cardioids, just them and no others in the mix, no compression or EQ. I didn't notice any difference in sound quality between them. It occurred to me only later to ask Schoeps about the diaphragm replacement. Of course, the diaphragm had been replaced with Mylar.
No doubt careful measurements would have revealed some small response differences. Particularly if you subtract the one signal from the other, almost any two capsules--even a factory-matched pair of recent manufacture, using identical materials and manufacturing methods--will show some difference.
But my point is, during the weeks when I wasn't aware of having one capsule with Mylar and one with nickel, I never heard a difference. And I definitely listened to these recordings critically at the time.
For what it's worth.
--best regards