After many many years of not playing anything, I dug my Sigma SDM-18 out of storage and brought it to my local guitar shop (shout out to Noll Guitars in RI). After a fresh set-up, strings, etc, she sounds gorgeous.
Here's what I know about this guitar:

-I bought it new somwhere in the mid 80's, certainly no later than '88.
-It has "Sigma Guitars est. 1970" inlayed on the headstock.
-Stamped inside on the back brace is: 286982, this is followed by an oval shaped stamp which reads "Sigma Guitars made in (?unreadable)for CF. Martin & Co", followed by "SDM-18". Also inside the guitar on the base of the neck is another number which reads: 804158.
-By looking at the edge of the soundhole, I can see the woodgrain countinuing through leading me to believe that it is a solid top.

From what I've picked up in this most excellent forum is that:
A. The "S" in SDM-18 may stand for "solid", "D" means "Dreadnought" and "M" means "Mohagony"
B. The stamping on the back brace, as opposed to a paper sticker, gives it a build date in the 1980 - 1984 range.

Anyhow, I love the sound and playability of this guitar. It has inspired me at 46 years old to take some formal lessons and learn to play her for real.