Serial Numbers vs. Factory Order Numbers
For the most part, prewar Gibson banjos have factory order numbers rather than true serial numbers. While serial numbers were placed on instruments at the time of shipment and were intended to be used for warranty issues, factory order numbers were issued at the time that a production order for an instrument or group of instruments was sent to the production department. Factory order numbers were intended solely for in-house use to help keep track of instruments and costs during the production process.
All instruments manufactured by Gibson prior to and during World War II were assigned factory order numbers, although in some cases these numbers do not appear anywhere on the instruments. Some Gibson instruments, particularly higher-end guitars and mandolins, were also marked with true serial numbers. Most Gibson banjos, except for those manufactured in the early 1920s and in the late 1930s, were not assigned serial numbers. Most prewar Gibson banjos are marked with factory order numbers, but in the cases of some lower-end banjos there may be no number visible at all.
While serial numbers on prewar Gibson instruments are reliable indicators of shipping date, the same is not true for factory order numbers. While factory order numbers can be used to determine an approximate manufacturing date, it was not uncommon for instruments to remain unsold at the factory for many months or sometimes up to several years. In the case of instruments marked only with factory order numbers, shipping date is best determined by examining the particular instrument's specifications in conjunction with surviving company records when available.
Unlike some other companies such as C.F. Martin, Gibson's use of their serial number and factory order number systems was highly irregular and is still not fully understood by historians and collectors. We owe much of our current understanding of these issues to the research of Joe Spann, author of Spann's Guide to Gibson 1902-1941.
If you are searching for information about your vintage Gibson banjo, whether it has a number or not, I would love to hear from you and I'll tell you what I can based on my twenty-plus years of study of these fascinating instruments.