Factory Order Numbers (FON) with a letter from 1935 to 1941.
The 1st digit will be the year of creation; the other digits will be the production number.
You can even look for a 2nd number on the headstock backside of guitars built earlier than 1961. The Factory Order Number will include in it, a letter, which will refer to the year, after which a 4 digit number then a 1 or alternatively 2 digit number.
The Gibson Serial Number Decoder currently supports 6 formats from 4 Factories.
For guitars made prior to 1977 use the extended search function.
This new function will try to match the serial number against older formats, details required for an exact match are listed in yellow.