Most dating mistakes are not reading errors: they are looking in the wrong place, or reading a number that was never a date. This page lists the eight places a date actually hides, and the caveat that belongs to each one.
On the chassis or rear panel — stamped into the metal on Fenders, on a plate from 1969 on Marshalls, on a decal near the mains inlet on later Mesas, and on the chassis of Oranges rather than the wooden shell.
Front-panel or vent-panel numbers are often not serials at all, and a serial from a no-data era will not become a date.
Glued inside the cabinet of most Fender combos and on the head cabinet of piggy-back models.
It dates the cabinet batch rather than assembly, and the same alphabet was used again on reissues from 1990.
On the bell end or the side of the transformer can, sometimes underneath it.
It dates a part that could have sat in stock for six to twelve months, or been replaced decades later.
On the basket rim at the back of the frame; on the magnet cover of some Jensen units, and under the magnet bell cover on many Gibsons.
It dates the speaker, and on a British amp you should allow three to four weeks or more before the finished amplifier was tested.
On the back of the potentiometer case, reachable only with the chassis out of the cabinet.
Pots are frequently date-coded ahead of use, with post-dates of up to 18 months documented, so a pot is never the only witness.
An ink stamp on the inside of Fender chassis from the mid-1960s to the early 1980s.
The direction of reading flips after 1979, and the stamp can be missing or over-stamped by later service.
On the rear of the chassis or head shell: Fender QA stickers from 1990, Marshall barcode labels from October 1992 and date bars from August 1997.
Both are modern markings; on an older amp their absence carries no information at all.
Written by hand between the preamp tubes on the underside of the chassis on pre-1990 Mesas, and next to the serial decal from 2008–09.
It can require removing the chassis, and some models carry no born-on date at all.