I’ve been a guitar player for a long time, this mandolin has just been sitting in the case for years because I never really got around to practicing/learning mandolin...
My understanding (and what I hear from my mandolin-player colleagues) is that Sigma Guitars was a `budget’ brand established by C.F. Martin & Co. in 1970 to compete with the influx of imported instruments, and that while initially launched for guitars, Martin expanded the line to include mandolins.
From the internet -
“Brand: Sigma. The absence of a name on the headstock usually means the logo has either worn off, been deliberately removed by a previous owner, or it was an early "prototype" model.”
“The Label: The label or center-brace stamp indicating it was made in Japan and distributed by Martin is entirely authentic. Origin and Dates: Sigma instruments were built in Japanese factories (such as Tokai Gaki) under strict Martin specifications between 1970 and 1983. Model Varieties: Sigma produced a few variations of mandolins—most notably A-style (tear-drop shape) and F-style (with scrolling).”
I have had several Sigma guitars over the years, and they have always compared very favorably to regular Martins in terms if sound, playability, quality, you-name-it. Martin is a great company.