White introduced the 4-150 in the fall of 1973, but didn't go into full production until 1974.
White built the proto type 2-105, 2-150 and the 4-180 in 1974. They were introduced at the Farm Progress show at Fort Dodge, Iowa in the fall of 1974. There were some 1755's, 1855's and 1955's built in 1975, before they went into full production on the new models. The same holds true on the later models. The White 2-70 and the 2-85 were introduced at the Farm Progress show at Galesburg, Ill. in the fall of 1975, but didn't go into full production until the next year. The 1555 and 1655 were still being built in early 1976.