Hi. It's not quite correct that Schoeps "doesn't support" the MK 26 (three-pattern) capsules, but the repairs which they can offer are limited, and the omni and cardioid settings of the capsule are unlikely to be functioning correctly at this point. In quite a few cases the figure-8 setting can be made to work reliably, but not the other two patterns. Unfortunately, in some cases the capsule can't be worked on at all without a real risk of losing whatever functionality it still has left.
All other M 221 capsules (single-pattern or two-pattern) can generally be completely repaired or restored as needed.
The M 934 C is essentially a modern MK 5 capsule. Some of them even had both designations engraved on them. Operationally the main difference is that the cardioid setting of the "C" model doesn't require (and shouldn't use) the "phase ring" which is required for the "B" model. You can see photos of both models on
www.m221.de . The attached front page from a 1955 Telefunken sales brochure shows an M 934 capsule with the phase ring in place.
Be aware that there are M 221, M 221 A, M 221 B and various "M 221 F" models. Depending on the suffix letter or lack thereof, different capsules may be needed, so if you're putting together a pair, you want them to both be from the same series. Also, be aware of the Hungarian fakes--they're rather common on eBay and in the inventories of "vintage" equipment dealers. The capsules are usually genuine but the bodies aren't.
--best regards