Dating Fender Tube Amps by Serial Number, Part I by Greg Gagliano Copyright 1997, 20th Century Guitar Magazine. Well, we'll get to good parts, but first a little background information is in order.
His particular area of expertise is in Fender amps made from about 1960 through 1967.
Devin runs Doc's Music in Springfield, Missouri and his interest lies in the woodie and tweed Fender amps made from 1946 through 1960.
These 1965 codes begin with the letter "O." The code for the year 1966 is "P." However, someone in the factory apparently forgot to switch the stamper from "O" to "P" in January 1966.
Therefore, amps from January 1966 have the date code "OA" (A denotes January) in green ink.
The factory realized its mistake in February because these amps have a "PB" date code in black ink.