Update 1:
Currently there is an auction
http://cgi.ebay.com/Gibson-1963-ES-345-Used-In-Back-To-The-Future-Part-II_W0QQitemZ230015480427QQihZ013QQcategoryZ33040QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
for the guitar he used in BTTF2 which is an ES-345