P.S. - I forgot to add three more observations in my previous post :

1. the end of the fingerboard of the GCS-6 with serial number 5628 that was recently sold on eBay has a rather prominent curve, whereas the fingerboard of my example does not have this curve;
2. The headstock and the neck of the Sigma with serial no. 5628 have a GLOSSY finish, whereas the finish of my Sigma's headstock & neck is matte;
3. All Sigma's from the early 70's that I've seen so far clearly seem to have a rosewood fingerboard and bridge, but oddly enough the fingerboard and bridge of my Sigma almost appear to be made of EBONY, as the wood has a really dark (almost black) appearance...