The tail rotor is more efficient if it is turning in the opposite direction of the main rotor tip vortex. The earlier designs did not take that into effect. Bell flopped the T/R on the 212/UH-1N so they would not have to redesign the gear box to rotate the "correct" way. The Bell aircraft which were designed from the mid-sixties on (206, 222, 407, etc) had gearboxes that turned the proper direction. The AH-1G had an MWO that flopped the T/R with mods starting in 1970.