very little info out there on this if at all.
sigma made acoustics yes and according to Wiki, in several Japanese factories until 78.
this has the correct logo on the headstock and could be Tokai made or not. here some others.
https://reverb-res.cloudinary.com/image/upload/a_exif,c_limit,f_auto,fl_progressive,h_620,q_75,w_620/v1398038597/yje9fcvdtqsuevy9hgee.jpg
http://d2ydh70d4b5xgv.cloudfront.net/images/c/1/1979-martin-sigma-electric-guitar-vintage-sg-copy-made-in-japan-natural-color-65eb240ff9f368bcb49ac734e0bc3c89.jpg