A debate over who was really the Fifth Beatle has raged since the Fab Four's earliest days. We sort through the leading contenders' resumes in the latest edition of You Think You Know Classic Rock?

Among them are key figures like George Martin and Brian Epstein, but also dark-horse candidates including Murray the K and Clarence Walker. Martin, of course, worked as the Beatles' principal producer. The late Epstein, meanwhile, helped shape the group's image, got the boys a record contract and served as the members' early sounding board in business affairs.

There were also several figures who actually shared the stage with George Harrison, John Lennon and Paul McCartney, giving credence to their candidacy as the Fifth Beatle.

The late Stu Stucliffe, for instance, was the original bassist in the Beatles' club-playing, pre-fame days. Pete Best served as the group's co-founding drummer, while Jimmie Nicol took over for an ailing Ringo Starr during a portion of the Beatles' 1964 world tour. And Billy Preston was the only other artist to receive credit on a Beatles single, with "Get Back."

Then there was disc jockey Murray the K, who gave the Beatles an early boost on his New York radio show – and who subsequently took to calling himself the Fifth Beatle. The above video highlights a few others who might qualify, as well. If you're still undecided, McCartney even weighs in on this lingering debate.

But, wait ... what about Clarence Walker? Who's Clarence Walker, you ask? We have definitive proof of his early involvement as a saxophonist in the Beatles, via a 1962 photograph provided by Walker himself "before they kicked me out of the group."

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You Think You Know the Beatles?