I have a Sigma DM-18. It is stamped Made in Japan by Sigma for C.F.Martin and Co. Serial number S 42319. I purchased it new in 1981. I have no idea when it was made. It is the best tonal acoustic I have ever owned.
I read somewhere once that Martin started building Sigmas in 1970. I forget exactly when they started in Japan; however I do remember that they moved the Japanese operation to Taiwan in 1984. The article said the quality suffered with the move. I played a couple of the Taiwanese Sigmas. They fall extremely short of the Japanese built ones.
I haven't payed any attention to the model numbers, years they were built, etc.; but if you find out when/where it was built, and it was built in Japan, I'm sure you'll be happy with it.