Dating a pocket watch is rarely one clue on its own. I work through serial numbers, hallmarks, the case maker’s mark and the movement grade, and only trust a date once two or three of them agree.
Serial numbers
The movement serial, stamped on the bridge or plate, is the most reliable starting point. Manufacturers such as Waltham and Elgin published serial number ranges by year, so a serial reading in the low millions might place production within a specific two or three year window. The case and the movement were sold separately in this period, so the case serial tells you nothing about the movement’s age and should never be used on its own.
Hallmarks and case makers
A silver or gold case carries assay office hallmarks: a town mark, a standard mark for the metal fineness, and a date letter. The date letter cycles through the alphabet in a set typeface and letter case, so it can be read against published tables for the assay office in question. The case maker’s mark, usually a set of initials in a shield, narrows things further and can confirm the case was made close to the date the hallmark suggests, rather than re-cased later with an older movement.
Movement grade
The grade name and jewel count, printed on the movement, point to a specific production run within the maker’s catalogue. A grade introduced in a known year rules out anything earlier, even if a serial number is worn or illegible. Taken together with the case hallmark, this is usually enough to give a date within a year or two, and to say with confidence whether the case and movement started life together.