Hi,
I have a Sigma 52sgcs-4. It was made in 1973. It is still in excellent condition. The sound has improved over the years. I always thought this was a Great guitar. I have not been able to find much info on the net about this model. I have noticed that this guitar does not have the adjustment rod in the neck, so I have been very careful to control the humidity level that I keep it in. Supposedly the best level is in the low 40%.
My understanding is that Sigma is an off shoot of Martin. Backed up by the fact that Martin puts their name on it too. They are made in Japan (to the best of my knowledge). I'm no expert but this guitar has served me well over the years.
I recently bought another Sigma. A "DM1stce" it came with a very nice "Martin hard case". Got it at "8thstreet.com" for $380.00 American-delivered. This one is an electric acoustic with the cut away body style.
A couple of my buddies have let me do a closed eye comparison to their guitars (one a Martin DCX1e and the other was an Ibanez AEF37E)
both electric acoustics. I could not tell the difference between the
Martin and my Sigma. The Ibanez (and I was really surprised by this)
I could pick out every time simply because the tone was more muffled than the other two.
As I said I am by no means an expert. But for the money Sigma is a good low to middle price range guitar.