Same format, but first digit and fifth digit both same number. Example: 91234567 = 2009? No — 91234567: digit1=9, digit5=6? Mismatch. So invalid.
Typically found on a paper label inside the body, visible through the F-hole or soundhole. Some modern acoustics also have them on the back of the headstock. 2. Era-Specific Decoding Methods Modern Era (1977–Present) gibson serial number
YDDDYPPP