Four actors have played Gordon on Sesame Street since 1969: Matt Robinson, Hal Miller, Roscoe Orman...and someone else. This video shows footage from an early test pilot of the show. Producers have attempted to identify this actor who is portraying Gordon, but have not been successful. It's a mystery, and the whole Sesame Street community is trying to solve it! Click on the link to study the clues.
Link -- Thanks, Dan Lewis!