Guitar / ES-335 / ES-345 
It's been said on this page that his 55's guitar was an ES-335, but it could be an ES-345 with aftermarket bigsby. Or possibly an ES-335 in the first film and ES-345 in the sequel. Doing some research at www.bttf.com/forums should hopefully give me more insight and then I'll change the trivia. I'm a nerd.