Well, Clayton Moore as the LONE RANGER used this ploy quite often and that show started up in 1949, so I think it wins the award. Another show that used the disguise plot device a good bit was WILD, WILD WEST by Jim West's friend, Artemus Gordon. But that show's first season began in 1965.

Gary "I'm pretty sure the Lone Ranger wins this one" O.

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"Do not challenge supernatural unless armed with sword of truth" - Charlie Chan at Treasure Island