... and to address the emphasis of the thread on pricing: The CMT 30/40/50 series ended in the 1970s (except for the CMTS 501 stereo microphone, which was produced into the early 1980s), and back then Schoeps wasn't as widely known then outside of western Europe as it is today (it took me months to find out where I could buy them), and they were strictly a high-end professional product--the "prosumer" market didn't really exist for most of that time. Thus Schoeps' production quantities were smaller during those years than they are now.
As a result there are far, far fewer CMT 30/40/50-series microphones on the used market than CMC-series microphones. The ones that have XLR connectors and are fully functional--especially the CMT 50 series which used standard 48-Volt phantom powering--can fetch prices in the same range as today's models, whether that is entirely rational or not. There is truly no sonic or technical reason to prefer any CMT-series microphone over its CMC-series counterpart, except that the CMT 50 series drew lower current (< 1 mA) than today's P48 models (~4.5 mA), and of course the CMTS stereo microphones were unique to the CMT series.
But the three-pattern capsules are a special case. Due to their lack of further maintainability, good ones even from the CMC series rarely come to market at all any more, and the prices are a bit of a wild card, as are the accuracy of the descriptions that you'll find on sites such as eBay.
The update to the gasket material occurred 10 to 15 years after all production of CMT-series capsules had already ended. As a result, the only "MKT 6" capsules that are fully functional in all three patterns, and can be expected to remain that way for a decent number of years, are those which happened to go back to the factory for maintenance during a rather specific "time window". So that's a minority (maintained during that time) of a minority (three-pattern) of a small minority (CMT series).