Just to keep the interest going, I've just bought for a very good price from TradeMe (the Antipodean EBay competitor website), an oldie-but-goldie Sigma 12-string. So, I've been using all the info on this thread to try and date it.

Here are the specs: Headstock: is inlaid mother-of-pearl "Sigma Guitars est 1970", (no sideways M), which (I think) dates it at later than 1980; interior stamp says it's a DR 12-28 model, which makes the body style the dreadnaught D-28, top solid spruce and the back and sides rosewood (R) - and very nice they are, too, only a couple of very minor dings. I've polished it up with Martin guitar polish and the wood just glows with that beautiful patina that old guitars get.

One stamp on the back-brace; "Made in Japan for G.F. Martin & Co"; and a separate stamp for the serial number S46 82. Which places it as no later than 1984, when they moved production to Korea and went back to paper labels.

I suspect that these items may place it at around 1982, which was during the time they were using Martin body design numbers.

Anyway, for all that it's over 25 years old, the neck is as straight as a die, the action is low and sweet, the volume of sound you can get out of it is awesome and the tone sounds very nice. Also it has an under-bridge pickup (which is still working) so I can put it through my amp when I play with my church music group. I'm glad I got it!

Incidentally, IMHO, the reason G.F. Martin shitcanned the Japanese operation was that they were getting a bit embarassed by the quality of the products: this one certainly sounds well equal to any Martin 12 I've ever heard and probably sold for about a third of the price!