To Dana    If yor guitar wears a "C" and is not a cutaway it probably means " custom" in Martin speak.
That will mean it could have abalone binding/purfling on the body/neck/fingerboard or all. Is this the case, or is there any other feature that is not familiar with Sigma's?
There are Sigma's that sport an "H" after the model # that signifies "herringbone" binding like the Martins.