Initial construction was in Japan, and those from 1970 to approximately 1978, were of solid high grade tonewood (top, sides and back)
but from then onwards changed to laminated construction for the back and sides. The first Sigmas were dreadnought acoustics, DR-1
through DR-7 (D for dreadnought and R for rosewood and the number denoting the grade, 7 being the top grade instrument). Some GCR-6
triple 000 and OM Series were built in Japan also. Sigma built the GCS (Grand Concert Series) to Martin's 000 body specifications.
There were also classical guitars in the C (Classical) series. Construction moved from Japan (1970-83), to Korea (1984-93/94), and
finally Taiwan (1993-2007).
In 1980, the model lines and designations were changed to capitalize on Martin model names: the DM-18, DM-19, DR-28, DR-28H, DR-35,
DR-41, DR-45, but the older model types were also continued as well.
Sigma guitars made in Japan from 1970 through 1979 can be distinguished by a headstock logo consisting of the single word SIGMA
surmounted with Martin's ("sideways M") logo in mother of pearl above it and an inner paper label giving model and serial numbers.
Some guitars in some years however, particularly the classical range, will not have the headstock logo, but all had the inner label.
In 1980 the paper labels were discontinued and the back brace was stamped. With the move of production to Korea in 1984 the paper
labels were reinstated but of a different design.
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.