Serial Number: There is no information readily available for dating Fender amps by serial number but there are people working on it. Circuit Code: In 1963 Fender stoped using model numbers to identify schematics and began using the model name and an assigned circuit number.These circuit numbers which are located on the tube chart can be usefull for getting a rough date on an amp.There are several ways to determine when a Fender amp was produced.
The earliest is a buried soil beneath the blown sand which contained Mesolithic to Bronze Age artefacts.
Here is a cool link to a Vintage Fender Amp price history chart that tracks sales data for specific amp models by the month.
Date Code: Wide panel, narrow panel, brown, blonde, and black face amps have a date code hand-stamped on the tube chart. The first letter translates to the year and the second to the month. Example: FG=July 1956, QA=January 1967, and a reissue CG=July 1992 EIA Source-Date Codes: Electronic components such as transformers, potentiometers, speakers, and some capacitors are often stampted with a date code indicating who manufactured them and when.
An assemblage of 763 flints included a few Mesolithic artefacts but was mostly of Late Neolithic and Bronze Age date.
A succession of ceramic assemblages included a small Middle Neolithic group (4 vessels), two distinct Beaker groups, one early (Lanting and van der Waals steps 1–3 (8 vessels), one late (steps 3–6) (45 vessels), an Early Bronze Age group of collared urns (43 vessels) and a Later Bronze Age group (26 vessels).
Fender installed casters on some larger amps and cabs beginning in 72.