I'm making the reasonable presumption that the last serial number for any given year is one less than the starting serial number for the successive year. The guide as such only lists the beginning serial number for each year. The source of the data is from a farm equipment quick reference guide that's intended to assist dealers in determining the year and current market value of trade-in equipment. Oh, and there was no 'F' for the either the 450 or 550. Since the JD450 was up to 'G' at this point, so the 550 would be elevated up to 'G' as well. I think Deere was trying to make things fit into a homogeneous group with the same variant letter, like Cat had been doing.
Note that the JD550 jumped from 'B' to 'G' here.
Here's the list of serial numbers, by year, for the JD550 through JD550G.
