I think it means starting at that serial number. If the machine is from the 70's or later the the first 2 letters are the year starting in 1974 I think. I've seen 1971 and 1972 machines that have 71 and 72 prefix to indicate the year. 1973 started with an H (1970's) and then a letter for the year D. A Jan. B Feb. C Mar., etc. They didn't used vowels. Should still be able to find a serial number list online. J is 1980's and K is 1990's. As far as manuals we bought a 1953 GMC 3/4 ton or 1 ton (single wheels) for $150 at an auction. It had a wooden grain box. The guy that drove it home for us said he could steer with one hand. It ran like a top! It even had the original owners manual in the glove box! It told you how to do all kinds of stuff like adjusting valves and other stuff unlike newer manuals that just tell you to take it to a dealer.