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.