Austin - ya Frank's got a good suggestion on adding a 149 which were built from 1971-74. If you could narrow down when your 55 was built you could match up to the same month/year for the other unit. (IH introduced the Quiet Line series late 1974, I think Oct/Nov so if your 55 ended up being late 74 you might have to match it to a 1450 or similar Quiet Line unit).