Schoeps introduced the MK 2, the MK 3, the MK 4 and some other capsule types all at the same time decades ago. "MK" definitely doesn't stand for "Mark"--if Schoeps had named their products on that basis, the MK 3 would have been the improved replacement for the MK 2 and then the MK 4 would have superseded the MK 3, etc., but it's nothing like that.
So just say the letters. Together they stand for the German word for "microphone capsule" (Mikrofonkapsel).
--best regards