Just purchased a Sigma CS 5, $80, 8 out of 10. I have just read the entire post. There was not much information on dating by the label.
The soundhole label is shiney white with a violet border.
It reads; E (SIGMA letter in violet)
         SIGMA (in black)
      CF MARTIN & COMPANY
       Model CS 5
    Serial NO. 000073   N
    Manufactured in Japan

I'm guessing this is an early '70's guitar?