If memory serves '94 was the year they started laser engraving serial numbers right near the stamp, so that would make this pre-'94.
Here's where I really start guessing: I think at first there were no K Dark Medium-Thins. All of them were Thins, so there was no need to ink that weight on there. I had a few in the early '90s that said Medium-Thin, so based on that alone, I'd guess mid-late '80s to '90 or '91-ish.
Nothing to do with the year, but I thought the weights were interesting on these current K's:
http://memphisdrumshop.com/cymbals/...crash/cymbal-series/k-series/cymbal-size/17"/
Yours is lighter than any of the 4 currently listed thins and medium thins from there.
I have an 18" Zildjin Dark Crash I bought in 1990.
It's actually a thin, but no "Thin" mark on it, as this is before the "medium Thin" was introduced.