LONDON.- Somewhere, an aging drummer (identity unknown) is probably still kicking himself. A newly discovered letter found folded in a book at a Liverpool yard sale has shed new light on the Beatles’ early days, revealing that Paul McCartney offered an audition to a mystery drummer in 1960, just a few days before the band left for a formative two-month gig in Hamburg, Germany. The letter, to be auctioned next month by Christie’s, has surprised Beatles scholars. It was written two years before the band bounced drummer Pete Best in favor of Ringo Starr, who arrived just in time to help the Beatles’ conquer first England and then the world, earning untold millions along the way. The Aug. 12, 1960 letter handwritten by McCartney offers an audition to someone who had advertised their availability in the