"Secret Chains" from Secret Six vol.1 #4 (DC, 1968) by E. Nelson Bridwell, Joe Gill and Jack Sparling
Too short-lived to become the espionage equivalent of the Sea Devils, Time Masters, etc., facing giant apes and UFOs, the original Secret Six had a premise worthy of today's hit television programs. They were six specialists blackmailed into working together by the mysterious Mockingbird, the twist being that one of them was actually Mockingbird. The series didn't last long enough to reveal who it was, so it's still a mystery. The Action Comics Weekly strip did give an answer years later, but it apparently wasn't the one Bridwell had in mind. I say the mystery persists then. Unless someone knows if he ever gave up the answer?