You shouldn't settle for anything < S-15 or newer for the Gemtech MK-9K, if that's what you really want. As far as I've seen, Gemtech's supply to distributors this year have all been date coded S15. I went with S15 x2, both cleared form 3 and were delivered to my local SOT a couple weeks ago:
(S12 was the last year for the UMP .45 QD from Gemtech)
Changes for their latest MK-9K includes new packaging design I was told. For any of the latest revisions, I doubt even Gemtech could answer such specifics since they seem to be the least knowledgeable about their product, but there have been many changes to their 3-lug over the years (total of 5 variants pictured in this thread).