To give an example, a typical Oxford speaker from the ’60s will look something like: 465-217.
465 designating the Oxford EIA code, 2 designating the year 1962, and 17 designating the 17th week of ’62.
If three digits are present, the first digit would refer the units of the year i.e. If four digits are present, the first two digits refer to the year i.e. The last two digits would refer to the week of the year i.e.
("26" would mean the 26th week or approximately June).
Remember, your amp is newer than the newest component.
For example, if you find pots from late ‘64 and transformers from early ’65, you can be pretty sure your amp is a 1965.
Take a look at this ’62 Fender Princeton Amp, and you’ll see that exact speaker, as well as the typical Schumacher transformers (EIA 606) used in Fender amplifiers.
The EIA code will consist of 3 numbers followed by a date code of 3 or 4 numbers designating the year and the week.
Some things are very obvious such as non-original or reconed speakers, non-original transformers, replaced pots, re-tweed, re-tolex, re-grill, etc.
and these changes are often disclosed and of a non-malicious nature.
Other things to look for include chasses placed in cabinets from a different year, “doctored” tube charts, non-original control plates (usually reproductions) on silverface amps, original transformer bell ends (they have correct date codes, of course) on non-original transformers, and non-original knobs (either repro or silverface knobs on blackface amps).unusual things can be found such as the empty “Pulse Adjust” hole on the rear of early ’60 brown amps, the “middle” volume control, use of tweed style grill cloth, strange non-documented transitional circuits, and changes in tolex color including the super-rare cream colored “brown” tolex that is found on some late ’60 amps. Given that people may refer to this information seeking specific production quantities of amps they are curious about, it should be pointed out that the serial numbers apply to chassis types, and not specifically to amplifier models.
Looking at serial numbers next to the ’60 5G5 brown Pro Amp for example, we see numbers ranging from 00001 to 02000, suggesting that there are 2000 of these amplifiers made in ’60.