Trilon

A trilon is a triangular prism-shaped object, turning on an axle (either mechanically or manually) to show different letters or images. Trilons have been used on game shows and billboards.
The squares on the original Concentration game board, first aired in 1958, may have been the first use of trilons on a game show. The game combined the card game with a rebus puzzle, which was gradually revealed as matches were made. As the show was an immediate replacement for the disgraced Twenty-One, the network insisted on keeping the puzzle pieces under high security, attaching them only to the trilons as needed.