Turn a vintage Seiko over and the serial number tells you more than the seller might. First digit: the last digit of the year. Second: the month, with O and N standing in for October and November. A 6119-8083 with a serial starting 21 left Suwa in January 1972. No paperwork required, no faith in anyone’s memory.
European casebacks keep different records. British import hallmarks date a case to the assay year. Swiss cases carry key numbers and patent stamps that pin down a reference when the dial has been swapped or the papers are long gone. None of it is secret; all of it is checkable, which is the point.
The scratches inside
Open the caseback and you’ll often find tiny scratched codes on the inner surface: dates and initials left by every watchmaker who serviced the piece. Three or four sets of scratchings is a good sign: it means somebody cared enough to pay for maintenance, decade after decade. A pristine inner caseback on a sixty-year-old watch is the suspicious one.
When we list a watch, the reference and serial have been read against each other. If a case and movement don’t agree, we say so, or we don’t sell the watch.