Jimmy Page did not play on Traffic's "Mr. Fantasy." That was definitely Dave Mason. This is one of those rumors like "Jimmy Page played the riff for 'All Day & All of the Night!'" Nope. Jimmy Page did a lot of session work but by the time "Dear Mr. Fantasy" came out, Jimmy Page had quit doing sessions and was deeply involved with the rapidly dissolving Yardbirds. He wouldn't have had the time. Traffic for their part had been rehearsing by themselves in a house in the English countryside, and would not at all have been interested in letting any session musicians sit in on their carefully created work.