According to Jim Dunkin's book, it was made in 1972, and was actually the first version of the X15. They cut the G15RV case off above the bypass and bolted on a separate cylinder jacket. By 1973 they had a "proper" one piece case, but the casting featured an unused rectangular front intake boss ala the G15. The 1972 version had the glow plug offset to the front of the engine, and in 1973 it was moved to the center.