You can learn just about anything on the Martin Forum.
I'm johnlg over there.
This is the wiki entry........
In 1970, guitar maker Martin created a line of inexpensive guitars made in part from laminated wood, instead of the solid wood used in their main line. (The name may have originated by turning the Greek letter Sigma (Σ) on its side.) Construction moved from Japan (1970 - 83), to Korea (1984 - 93/94), to Taiwan (1993 - 2007). The first Sigma's were dreadnought acoustics, DR-1 through DR-7 (D for dreadnought and R for rosewood). Sigma built the GCS (Grand Concert Series) to Martin's 000 body specifications. There were models in the CS (Classical Series). Other instruments included mandolins, banjos, acoustic and electric basses and solid body and hollow body electric guitars. Some solid body electric guitars were made by Tokai Guitars Company, LTD. After finishing, "Each Sigma instrument was brought into the Martin factory to be inspected and adjusted by Martin personnel before going to the authorized retail store for retail sale.
The Sigma guitar line was discontinued in 2007.
They were good imported guitars. I had an 18 series Sigma of some sort many years ago. I paid about $190 for it.