If you are unfamiliar with thegreengmthand.com, it provides some very useful information for the novice and seasoned Rolex collector - especially the page identifying the Rolex crown markings.
Below is a link to their page identifying the various crown markings that identify the trip lock, twin lock etc. as well as the precious metals.
http://thegreengmthand.com/about/useful ... variations
Once you arrive at this page, scroll completely down until you find the header that reads:
Rolex Crown Dots: Different Designs To Indicate Different Metals
If you go to the top of this page which has an index header that reads "Useful Rolex Knowledge" you'll find another link for further crown identifications titled "Rolex Crown Differences: Twinlock Crown vs. Triplock Crown"