British assay marks usually combine a standard mark for the metal, a town mark for the assay office, a date letter for the year of assay and a maker's or sponsor's mark. Each element is read against published tables for that office.
The date letter is the most useful and the most easily misread: letter forms, shield shapes and cases changed on cycles that differ between offices, so a letter alone is ambiguous without its town mark and punch shape.
Continental practice differs. French, Swiss and other systems used their own standard and control marks, and import marks were applied where a foreign case entered a hallmarking jurisdiction — evidence of where a watch was sold rather than made.
American cases were generally not hallmarked in this sense. They carry case-maker trademarks, metal statements such as gold-filled warranties, and their own serial numbers, which are unrelated to the movement's number.
Treat the result as two dates. A case assayed in one year holding a movement made in another is common and not in itself suspicious; state the two separately and say whether the pairing is verified.