British assay offices struck a date letter, a standard mark, a town mark and a maker's mark on precious-metal cases. Where the marks are legible, the case can usually be dated to a single assay year using published tables.
The movement is a separate object. In the English trade, movements were finished by one shop, cased by another and retailed by a third; a movement could be cased months or years after completion, and old movements were re-cased throughout their working lives.
So the hallmark gives a limit, not an identity: the case was assayed in a particular year, and the movement it now holds may be older, contemporary, or later if the watch has been re-cased.
Check consistency. Does the movement fit the case without spacing rings? Do the dial feet match the movement? Is the case configuration one that the movement's setting system implies — a hunter case demands a movement with the correct winding and hand-setting orientation?
Other traditions provide different evidence. Swiss and French cases carry their own assay and import marks, American cases are often marked with warranty terms and case-maker names but no date letter, and gold-filled cases carry warranty periods rather than dates.
Where evidence conflicts, record the conflict. A catalogue entry that says 'case assayed 1898; movement configuration consistent with the 1880s; relationship unconfirmed' is more useful than one that asserts a single year.