The older watches dont have it or do they .. but the new watches have the 2 prong fork symbal .. what does it represent and why did they start using it
It's a stylized tuning fork. Bulova introduced the Accutron Watch in 1960, which used a tuning fork to drive the mechanism (gross oversimplification?) of the watch. The 'fork' became the symbol of the Accutron line and is currently used as the Bulova corporate logo. Bulova still uses the Accutron name for it's watches but no longer produces tuning fork movements. There are volumes written on the Accutron line of watches. An internet search will bring you hours of reading if you're interested.
Have a look on this page: http://www.mybulova.com/vintage-bulova-catalogs
Scroll down to the Facts About Accutron(1964). There's a few pictures that show you what Geoff is referring to.