Hi all - since no-one else seems inclined to answer the spate of recent Guest postings about their Sigma finds, let me try...

1. Serial numbers of early Sigmas (70's to 80's) are of no help. It would appear that Martin didn't have any consistent system and their contracting guitar workshops, firstly in Japan, later in Korea (and there were quite a few over the years) probably used their own numbering systems, or just made them up. No-one to my knowledge has ever attempted to classify the serial numbers in a way which would identify the place or year of manufacture and I suspect that it isn't going to be possible now. Later models might, but I don't know of any sources for this. Serial numbers can sometimes confirm things: there are at least three Sigma 12-strings in New Zealand I know of with close serial numbers, which indicates that they may have been part of a batch imported here at the same time.

2. Model designations were sometimes stamped on the back brace and sometimes follow the Martin model designations. Mine is stamped "DR28-12", which means that it's a 12-string D28 body with rosewood sides and back. But sometimes they don't.

3. There is usually a "country of origin" brand on the back brace. This is oval, and is in three parts, the top lettering reading "Sigma Guitars", the middle the country (Japan on mine) and the bottom "C F Martin". Lacking this brand, the guitar probably isn't authentic (but who knows, really!).

And finally, Guests, do your homework! Read through all the posts on this thread. Yes, all 600-plus. You might learn something. The Sigma story is an interesting piece of history and learning about it will add to your experience when you pick up your guitar. Good luck - if you have found a nice old Sigma, look after it and it will repay you with many years of music.