How the code dates an instrument
A potentiometer or speaker carries an EIA source-and-date code: three digits for the maker, then the date. A seven-digit code reads maker, two-digit year, then the week of the year; an older six-digit code has only a single year digit, so the decade has to come from the instrument itself. The pot makers are CTS (137), Stackpole (304), Centralab (134), Allen-Bradley (106), Clarostat (140), Bourns (381) and IRC (615); the same code is on amp and hollowbody speakers, so Jensen (220), Oxford (465), Utah (328), Rola (285), Altec-Lansing (391) and Electro-Voice (649) decode too.
The date is the pot’s, not the guitar’s. A pot is made before the instrument is assembled, usually within a year, so read the year as the earliest the guitar can be. Where several pots carry different dates, the latest one is the tightest floor. Replaced pots and repairs muddy this, so weigh it with the rest of the instrument.
Where to find the code
Look on the back or the side of each potentiometer, inside the control cavity or through the f-holes. The code is a run of six or seven digits; ignore the resistance value and any other stampings.
When to use it
Reach for the pot code when the serial number cannot pin a year: a pre-1977 Gibson, a vintage import, a guitar whose serial is missing or worn. Start with the serial number dater, and when it says the number cannot date the instrument, the pots often can.