Determining the age of a Gibson Les Paul guitar unlocks its value--but it takes more than just checking a serial number. Serial numbers conflict and can be more of a hindrance than a help. Finding the guitar's true age can be like something out of "CSI," where you figure out a date based on a variety of forensic evidence--not just a serial number.
Look for an ink stamp. The serial number on Gibson Les Paul guitars is located on the back of the headstock. The earliest Les Pauls made in 1952 may lack serial numbers. Beginning in 1953, a three-digit serial number was stamped on the top. Later in the year, and continuing through 1961, Gibson stamped a five- or six-digit number in ink on the back of the headstock. The first digit is the year of manufacture; the remaining digits are the production number.
You may also find a second number on the headstock backs of pre-1961 guitars. This is the Factory Order Number (FON). The FON consists of a letter, which corresponds to a year, and then a four-digit number followed by a one- or two-digit number. These can help narrow down the year of manufacture but not much else, as they were assigned at the beginning of a production run.
Find the "brand" if the ink stamp is missing. From 1961 to 1969, Gibson used serial numbers that were stamped into the wood, not inked on. These numbers don't real follow a pattern. For instance, guitars numbered up to 42440 are from 1961, but a guitar numbered 042400 would be from 1967. The best advice if you have a Gibson Les Paul with a stamped-in six-digit serial number is to consult a reference book such as "The Blue Book of Electric Guitars" or the "Gibson Les Paul Book." They will be able to match up dates with numbers. In any case, write down the serial number. This will keep you from having to go back to the guitar time and again.
Narrow down Les Pauls with labels. From 1970 to 1975, small black labels containing serial numbers were placed on the headstock backs of guitars. The words "Made in USA" were also impressed on the backs (though some earlier models had those labels as well). The label changed to a transfer from 1976 to 1977 with eight-digit numbers. Numbers beginning in 99 are from 1975, numbers beginning in 00 are from 1976, and numbers beginning in 06 are from 1977.
Check the back for a stamped-in eight-digit serial number. If your Gibson Les Paul has an eight-digit serial number stamped on the back of the headstock, you're in luck. Your guitar should be relatively easy to date. Eight-digit Les Paul serial numbers follow a "YDDDYPPP" scheme where "YY" is the production year, "DDD" is the day of the year, and "PPP" is the instrument rank and/or plant designation. This isn't universal--some Custom Shop models have six-digit numbers -- but it should allow you to date almost all post-1975 Les Pauls.
Don't worry about an extra "2." A "2" stamped above the serial number indicates the guitar is a factory second--not that big a deal once a guitar has been played and certainly not an issue if the guitar has been refinished or had major repairs.
Look for the volute. You should try to corroborate the serial number on your Les Paul with some physical evidence. First, don't use easily changed parts such as tailpieces to date your guitar. Many early Gibson Les Pauls have had the unwieldy trapeze bridge removed. However, if your guitar has a volute--an extra bit of wood at the base of the headstock, added for strength--your guitar dates from 1975 or later.
Check the headstock shape and pitch. When Gibson reintroduced the Les Paul in 1968, the peghead was larger than the original's. You may need to have a Gibson with a known date to compare headstocks, but if yours has a wider headstock that narrows down the date to the 1968-1985 era, roughly. The headstock pitch on Les Pauls made from 1968 to 1973 was 14 degrees; headstock pitches on all others was 17 degrees. Shallower neck pitch means an early second-generation Gibson Les Paul.
Beware of forgeries. The Gibson Les Paul is the most forged guitar on the market. If the serial number doesn't match up with the physical evidence and you're looking to buy, ask for some provenance before you pull the trigger. With Les Pauls, it pays to be careful.