Just received this:
Hello Peter,
Thank you for contacting Martin Guitar.
The Sigma series was imported from Japan (later from Korea and Taiwan) and available for sale at an affordable price point from 1970 - 2007. All Sigma guitars are constructed with laminated (ply) backs and sides. The Sigma series also included banjos and mandolins constructed with laminated (ply) backs and sides. Martin Guitar did not keep Sigma serial number records. As such, Sigma’s and Mandolins cannot be accurately dated. The Sigma brand is no longer affiliated with Martin Guitar.
Unfortunately, we do not have any captured information for this Sigma instrument as we are no longer associated with them.
Best Regards,
Evan
Customer Service Representative
So, no direct answer, but he makes it sound like that ain't a Sigma.
The mystery continues.
Peter