Everybody knows about serial numbers and how unreliable they became by the mid 60’s. There is, however, another number in many older Gibsons that will be a big help in dating your guitar. The bad news is that it won’t help you during the period when Gibson was reusing serial numbers with reckless abandon-three, four or more times over the course of a few years. The Factory Order Number (FON) was used by Gibson from 1952 until 1961 as a sort of in house inventory system. At the very beginning of the construction process, a number was stamped in the body of the guitar that began with a letter prefix. They are usually visible through the treble side f-hole. They seem to be some kind of batch number but I don’t know of anyone who has truly deciphered the system. I can’t even tell you if the numbers are unique or whether they followed some “batch” numbering system. Where they are very useful is for guitars that seem to be at the end of one year or the beginning of another. During the period of 1959 to 1960 and, 1960 to 1961 there were a number of transitions going on in the ES series. You might have a guitar with a 61 serial number that still has a long guard, or a 60 with a big 59 neck or 59 type knobs (no reflectors) or even a combination of features that seems to contradict the year indicated by the serial number. This is especially true of 345s and 355s. Because they were relatively low volume sellers (especially 355s), they may have been
stored at the factory in partially finished form for a number of months. For example, I have an ES-355 with a short guard and a white switch tip-both associated with 1961. The factory order number is from 1960, however. The likelihood is that this guitar was built in 1960 but not fully assembled until there was an order for it which apparently came in sometime in 1961-hence the later serial number and later features. I had a wonderful blonde 1960 ES-345 that had a most un 1960 like big neck (although not as big as an early 59) and bonnet (non reflector knobs). It was an early January build and had a 59 FON. So, why didn’t it get the later knobs? Probably because the transition to the reflector knobs was still going on. When the guitars were assembled, I’m sure all the workers cared about was whether the knobs all matched. It was probably easier to grab the older type because you didn’t have to pay attention to whether they went on a tone pot or a volume pot -so my assumption is that was what they did until they ran out of older knobs. You will see amber switch tips into 61 but you will also see white ones at the end of 1960. I’ve seen 61’s with long guards and 60’s with short guards. These kinds of “anomalies” can call the originality of a guitar into question. Enter the FON to save the day. All most folks need is a reasonable explanation as to why a guitar doesn’t strictly conform to the accepted norm and an earlier build date (FON) with a later serial number can explain a lot. Here are the prefixes (they go in reverse) 1952=Z, 1953=Y, 1954=X, 1955=W, 1956=V, 1957=U, 1958=T, 1959=S, 1960=R and 1961=Q.