The mark that dates a unit is rarely the one on the front. This is where the archived marks are found on the machines themselves, and what each one is evidence of.
On the chassis: a plate, an engraved number, or a stamped impression. On McIntosh, Quad, Thorens and Revox this is the mark that can carry a year — read it with the model.
On Revox include the S or G prefix; on McIntosh keep the letters inside the number.
A paper or foil label with a plain assembly date. On Leak this is the practical dating evidence, and it does not involve the serial number.
Photograph it before cleaning: it is the first thing a restoration removes.
Three letters on a Hunts capacitor, or Mullard/TCC equivalents, dated by the week and the year digit.
This dates the capacitor; the unit is never older than its newest original part.
On transformers, pots and speakers: a source number, then the year and week. It is what dates most American equipment without a serial table.
The decade comes from the unit, never from the two-digit year alone.
On turntables: an ink stamp on the underside of a cast platter, or a stamp on the deck where the maker put one. On Dual this is the only factory date on most units.
The serial number on these decks usually carries no calendar at all.
On tape machines: month and year stamped on the motor rotors of a Revox A77.
The closest thing to a build date on these machines.
On kit equipment: the Allied/Knight catalogue number, or the Heath and EICO model listed in the catalogues.
The catalogue year brackets the product; the assembly date is not recorded.
Faceplate finish, badge, switch shape, Mk I versus Mk II, ribbed versus smooth chassis.
A version bounds a period and tells you which range a number belongs to.