The Super 55 was first introduced in the sumer of 1954. They were built till early 1958. Starting with the 1957 model Super 55 tractors there were nice improvements made in the hydraulics. The 550 was introduced in 1958 and replaced the Super 55. The 550 was made for a long time  don't remember exactly when without looking. There are changes in them as well. In 1959 the engine displacement changed from 144 to 155 cubic inches. Serial number plate is down by the clutch petal.