You best bet would be to take a lot of photos of the guitar - front, back, headstock, any labels, stickers, stamps and serial numbers and put them on the Vintage Sigma Guitars page on FaceBook. Also read all the FAQ's on that site and as many of the posts as you can manage. The resident Sigma gurus, Richard Boettinger and Kevin Dunham are very generous with their time and expertise and most likely can give you a reading on your guitar's provenance. Sounds interesting!