Hi Neil, no need to get worried about my sanity :), as I've been dealing in the field of vintage guitars (also the more obscure brands) since about 20 or so years, and I see it as a nice little challenge to find out a bit more about the history about these particular guitars, confusing as it may be...:)

Anyway, here's a pic showing the label of the GCS-6 with serial no. 5628 that was recently sold on eBay :
http://i115.photobucket.com/albums/n314/the55thbeatle/Sigma6E.jpg

I read somewhere that only the very earliest Sigma's had this particular paper label (and this sounds quite possible, considering the low serial number of this example), however note that the Sigma that was for sale here until last week had an even lower serial number (4747) yet had the same type of label as my example...

Confusing? Yes..
Interesting? Also yes!

Kind regards,
Stefan