I got the Sigma in 1988. Its a Japanese Martin. I probably play this one the most. Its got a rich, full bodied sound, and it plays nice. Sigma guitars were based on standard Martin models and were inspected at the Martin factory before going to market. I had initially thought the model was DM-1B, but now believe in is DM-18. The DM stands for "Dreadnaught Model". Update 4/2021 it's actually a DM-19
The top is solid spruce, stained brown. The sides and back are laminate mahogany.
Martin has since let go of the Sigma brand and it was acquired by AMI in Germany, so the old website listing this model is no more. There is some information here.
Between 1988 and 2020 this was my only playable acoustic and I played it all the time. It has a bold sound as you would expect from a dreadnaught and very playable action. In 2020 I expanded my collection significantly with smaller guitars better suited for playing solo, but the Sigma still stands out with the biggest and brightest sound.