Every result on this site is produced by one rule and reported with its method and its limits. This page states the rule, the way confidence is set, and what the reference refuses to claim.
The governing rule. Component codes set a floor for the build date, never the date itself: if a transformer was replaced in 1978 on a 1965 amp, the transformer is silent about the amp.
Transformer, speaker, pot and capacitor codes all share the same limit. Where no assembly sticker exists, at least two independent original parts must agree before a year is reported as supported.
An assembly-time mark (tube chart, QA sticker, date bar, born-on date) outranks a component code. Component codes outrank cosmetics. Cosmetics are only used for era and never for a year.
High confidence means a maker publication or a factory system. Medium means a large, long-standing community table or an official features list. Low means single points, forum evidence or a system that is documented as approximate.
Where a code belongs to two cycles, two decades or two owners, both candidates are returned with the differences to check. Marshall model codes and Fender reissue letter pairs are the two commonest cases.
Ampeg publishes two eras without a serial mapping, and Peavey, Mesa, Gibson, Silvertone, Magnatone, Sunn, Standel and Premier have no usable serial series at all. Those amps are dated from features, model eras and component codes, and the gap is named rather than filled.
Every table, entry and model row carries the address it came from, and the source registry lists them with their type: maker documentation, reference work, archive or community research.
A Fender tube chart can read January 1967 while the Schumacher transformer on the same chassis is stamped 1968, week 26. The archive then reports 1968: the amp cannot be older than its newest original part, and the tube chart either sat in stock or the transformer is a replacement.
See this reading for a tube chart →The evidence library →