The early 70s models (1970 -1975) can be distinguished by a more squared peghead, shaped similarly to that of a "spatula." The logo consisting of a gold decal with the single word in block lettering SIGMA surmounted with the Σ symbol (the Greek letter sigma), in mother of pearl.
In 1978, model designations were added to capitalize on Martin model numbers: the DM-18, DM-19, DR-28, DR-28H, DR-35, DR-41, and DR-45. Pegheads & Logos Sigma guitars made in Japan from 1970 through 1983.
This is especially true of unusual, special (sample/prototype models) and/or low production number models (e.g.: DT-30.) The 'Gold' label: The next generation of labels are transition labels that are white w/Sigma and three horizontal lines in gold ink, the Greek sigma in black and printed black text (Made for C. The Σ symbol now also in this same pink/purple color.
In 2011 the German company "AMI Musical Instruments Gmb H" purchased the rights to the name and relaunched Sigma Guitars again, this time being produced in China. The model and serial numbers were usually stamped on in ink, but some are known to have been hand-penned. These will have a 4, 5 or 8 digit (7*0XXXXX) serial number stamped or hand written on them. The 'Purple' label (c1972 onward): These are again white labels that had gold & black lettering but this time with a border, often pinkish-brown or violet in color.
82751 – 905001931 90501 – 940001932 Introduction of the three shot Auto 3 with shorter magazine tube and shorter forearm marked “Browning 3 Shot” 94001 – 960721933 96073 – 961431934 Receiver legend Now with BROWNING and John M.
Browning’s bust between “TRADE” and “MARK”96144 – 995001935 99501 - 1035001936 The "Sweet Sixteen" was released but limited to a test market.103501 – 105850*1937* Introduction of the “Sweet Sixteen” sale to the public.
This may be due to them being built in several Japanese factories at the same time with no coordination or tracking in the numbering system.
Following the list are the facts and assumptions which were used to create it.
Details about dates, transitions, exceptions, and other nuances can be found in the Shirley/Vanderlinden book, Browning Auto 5 Shotguns. 1909 1 - 32001910 to 1914 There is only vague info about 16 ga SNs in SV (page 74) for the time 1910 to 1914. "65101 – 827501930 Barrel Address (US market) BROWNING ARMS COMPANY ST LOUIS MO.