You know. I too couldn't figure-out why I sometimes saw "MK-XXX" and sometimes, "MC-XXX".
I felt stupid after-words, but it took Taylor Johnson to explain to me that you would use "K" instead of "C" to spell "microphone" in Russian, so "MK" is used for domestic sales. You would use "C" instead of "K" to spell "microphones" in English, so "MC" is used for foreign sales.