Serial numbers appear in different places on different makes and models of vintage guitars; they might be found inside the sound hole, on the back of the headstock or on the bridge plate, among other places. Serial numbers are of varying reliability in determining a guitar's vintage.
There appears to be little standardization in the inscription or placement of serial numbers between guitar manufacturers. The most famous American guitar manufacturers, including Gibson, Fender and Martin, adopted different locations for placing serial numbers and different systems for producing those numbers. For example, one collector describes the method of assigning serial numbers at Fender as follows: the guitar assembler at the time would just grab a serial number plate from a bin of plated numbers, out of order and attach it to the bridge plate.
Gibson guitars states that there may be a number of vintage Gibson guitars with the same serial number, since Gibson has adopted so many different schemes of numbering over the years. They have recently tried to dispel confusion by adopting a new 9-digit numbering system.
In seeking the serial number on your vintage guitar, check all around the guitar and then refer to a reliable collecting guide or the manufacturer's website to see what kind of information the serial number provides,