During this brief window, Gibson switched to a 9-digit format starting with YY (e.g., 14xxxxxxxx for 2014). These don't encode the day of the year at all.

Reverb has a built-in tool that helps verify the year of manufacture. Since Reverb is a marketplace, they have access to millions of sales listings, allowing them to cross-reference serial numbers with actual sales history.

Pre-war and immediate post-war guitars often feature a serial number stamped on the back of the headstock or on the label inside the body. These are purely sequential and require expert cross-referencing to date accurately.