Logging these to get an idea of the years of production.
1988
1991
1992
1993
1996
1997
Production may have begun in Korea as early as 1980 ?
And according to this article in Vintage Guitar Magazine, production partly shifted from Japan to Korea in the 80s I believe, but guitars were made in both countries for many years making this much more confusing.
"Hello All, Have a question regarding Korean made Sigmas. All the online info (which isn't much) I've come across dealing with Sigma manufacturing history states that Sigmas were made in Japan from 1970- 1983, moving to Korea 1984-1993/94. I own a Sigma DM 4, made in Korea, paper label, blue border, gold script, serial #8013108 that I KNOW that I purchased in 1981. Bought from Music Emporium USA, Rockville, Maryland. If, as everything I've found online states, manufacture in Korea didn't begin until 1984, how did I get this guitar in 1981? I realize some Sigma info is sketchy but these dates seem consistent. Anybody have any input?"
I have identical Korean-made Sigma DM 3, #121138 .I had thought it was mid-1980's, but you're probably closer to correct dating. Axe sounds great and no bulging or separating ! My "poor man's D-18" !