So just how accurate should a vintage mechanical watch be?
The short answer: it depends.
The accuracy of a vintage watch is at the ultimately at mercy of its original quality and construction, and accuracy perhaps owes even more so to the quality of its upkeep over the years.
A high-quality, chronometer-grade watch that was carefully maintained and cared for over the years by skilled watchmakers could potentially approach chronometer standards still today. However, a more inexpensive design that incurred a greater degree of wear and tear in everyday situations that wasn’t as conscientiously maintained might not nearly be capable of such accuracy.
While many of these old watches can be repaired and adjusted to run very accurately there are simply some that are old, tired, worn out antiques. Very often mechanical watches were sorely neglected especially in the post-1970 era when inexpensive quartz replacements could be had and watchmakers began getting harder to come by. Some vintage watches are lucky to be running at all after the way they’ve been treated for three or more decades.
But in general it’s perhaps unreasonable to expect a decades-old watch to perform like new, and a +/- 1 minute a day rate for most any 30-60 year old watch with unknown or even no previous service history isn’t too bad in my opinion, especially for watches that may not have been capable of chronometer performance when new.
And indeed, the definition of “chronometer” has varied over the years before the adoption of the modern -4/+6 sec/day Swiss standard under the auspices of the Contrôle Officiel Suisse des Chronomètres (COSC). And some makers like Omega in particular delivered their military watches to an informal but un-tested/un-certified “adjusted for chronometer” standard of +/-10 sec/day, so in that sense after 60+ years performance in the neighborhood of 30 sec/day is probably pretty reasonable.
And like one collector I know is fond of saying: “I’ve never heard of anyone being 15 seconds late to a meeting.”