Stan (Login Rolco) VRF Member from IP address 18.104.22.168
By my experience:
Sigma symbols on a Rolex dial indicate the indices and hands are gold (likely to be 18K). However, they do not mean that the dial itself is made of solid gold. In fact, Rolex has never (AFAIK) made a gold dial except the fully paved ones. The YM has a solid platinium dial.
Since 70s, or even earlier, solid gold indices and hands have been fitted to higher models like Daytona, DD, DJ. Not on the manual wind 6694 or AK. I think some of the 150X have.
The Sigma or "OM" notation on an Omega means that all the indices, hands and the dial itself are solid gold. On the other hand, there are solid gold dials without the symbols.