Mk is the written short form of mark. Here, mark means a version or model number, especially of a car, plane, or machine. The Golf Mk 2 is the second mark of that car — the second main version. People write Mk 1, Mk 2, Mk 3 and usually say “mark one,” “mark two,” not the letters M-K. You will see it in car ads, manuals, and hobby talk. Though actually, the same letters can also appear in Bible notes for the book of Mark, and older writing sometimes used Mk for the German mark (the old money). Those are different jobs for the same abbreviation. For most learners, the “version of a machine” sense is the one that turns up in ordinary English. It is writing, not a word you use on its own in speech.