Pot codes can help you determine the year your guitar or bass was manufactured, especially in cases where you can't find a serial number on the instrument, or the serial number still leaves room for interpretation. (If you can find the serial number, go to our extensive Serial Number Database to help you find your baby's born on date.)
Keep in mind there are lots of cases where a manufacturer such as Fender may buy pots in bulk and use them over several years, so you can't always count on pot codes to be exact, but they can help narrow down the date when taking into account other factors. CTS, Centralab, and Stackpole discussed below are the most common pots you'll find in early American made Fender, Gibson, Epiphone, Guild and some other guitars.
CTS Pots
Let's start with one of the most common pot codes you'll see. That would be a number beginning with "137..." This indicates a CTS pot. CTS's EIA (Electronics Industries Alliance) number is 137 and the numbers that follow indicate year and week of manufacture. The first two digits after 137 will indicate the last two digits of the year, while the next two digits indicate the week. So "1377624" would be a 1976 pot made in the 24th week of the year. Some early pots from the 50s may only use one digit for the last digit of the year.
Newer CTS pots no longer have "137." Instead you'll see a very long number and letter string that indicate all kinds of info if you're curious (see the Red Star Guitars website). Then hopefully, you should also see a 4-digit number standing alone. This is your date code where the first two digits indicate the final two digits of the year and the last two indicate the week (so that "1407" indicates a 2014 pot made in the 7th week of the year).
Centralab and Stackpole
These are the other two main USA pot makers you'll come across and luckily they generally follow the same scheme as CTS. Centralab's EIA code is 134, while Stackpole's is 304. After those numbers should be your year/week codes. The date codes are often four digits, but can also be just three digits, as sometimes an early 50s pot may use only one digit for the year. These codes can be printed either on the bottom or the side of the pot. Sometimes the numbers will be separated with a dash after the EIA number.
Some other three-digit American pot manufacturer codes you may encounter are:
106 = Allen-Bradley
140 = Clarostat
381 = Bourns Networks
615 = IRC
Regardless of the maker, the following two digits should give you the year, just as explained above. One thing you may encounter is numbers being covered with solder or other gunk. Hopefully at least one pot in the instrument will show the date code. If not, you may find yourself having to unsolder some wires to uncover the code.
Egen/Morganite
These two types of British pots can be found in Vox, Hohner and other guitars, most commonly from the 60s. Egen uses a two letter code with the first digit A through L representing the month of the year Jan-Dec. The second letter H through O represents the years 1960-1967. So a code of "AN," would be a 1966 pot made in January. It is possible some Egen pots switched it up and put the year BEFORE the week. The two letter code should usually appear on the side of the pot.
Morganite pots usually found in early Vox guitars use two different dating schemes. One scheme uses three numbers with WWY (week-week-year). The other scheme is similar to Egen with a two-letter format, but more often putting year before the month. Year codes run S through X, representing 1961 to 1966. Month codes run A through M for Jan to Dec (The letter "I" was excluded to prevent mistaking it for the number 1). Apparently Morganite also made some pots with the reverse, meaning month before year.
Another thing to keep in mind is you'll come across pot codes not just in guitars and basses, but also guitar pedals, speakers, and amps. Many of the same rules laid out here will still apply. Also, you'll find plenty of imported pots in non-USA instruments that won't have any such information and thus won't be much help.