British hallmarking on gold cases records a fineness standard (historically including 18, 15, 12 and 9 carat, with 15 and 12 carat withdrawn in 1932 and replaced by 14 carat) alongside a town mark, a date letter and a maker's or sponsor's mark, and where the full sequence is legible and correctly matched to published tables it dates the case, not the movement, to a specific assay year.
Continental European systems used different conventions: French gold cases carry poinçons (state control marks) indicating fineness and period rather than a date letter system directly comparable to the British one, and marks changed with political regime; Swiss cases carry their own control marks, and both French and Swiss marks require reference to specialist tables rather than assumptions carried over from British practice.
American cases generally did not use a date-letter hallmarking system at all. Solid gold cases are typically marked with a fineness such as '14K' or '18K' and a case-maker's name or trademark; gold-filled cases (a layer of gold mechanically bonded to a base metal) are marked with a warranty statement such as a guaranteed number of years' wear, which describes durability of the gold layer, not a manufacturing date.
Gold-filled and rolled-gold cases are frequently mistaken for solid gold by inexperienced buyers relying on colour and marking format alone; the wording and format of the mark ('gold filled', 'rolled gold plate', a fraction indicating the proportion of gold, or a warranty in years) distinguishes them from solid-gold hallmarks or fineness stamps, and should always be read in full rather than glanced at.
Marks can also mislead deliberately or accidentally: cases have been re-marked, marks transplanted from a genuine case onto a replacement, and export or import marks added later can be confused with original assay marks. A mark that looks right in isolation should still be checked for consistency with the case's construction, wear pattern and the movement it holds.
As with dial and movement evidence, record what the case marks actually say and what standard reference confirms about them, and keep that record separate from any statement about the movement's date or the watch's overall history; the case tells its own story and should not be allowed to silently stand in for the whole object's.