There is no fixed standard. As mentioned above, most modern movements have specifications. As you may know, the variation reflects measuring timekeeping in various positions.
@Archer started a great thread on this:
https://omegaforums.net/threads/tim...-understand-how-they-work.87293/#post-1128576
Regarding vintage watches, I think it's not always realistic to demand that they meet the same timekeeping standards that were specified when they were new, since they may not have been serviced regularly and various parts may have experienced significant wear. Yes, we have all had experiences where a vintage watch keeps excellent time, but not always, and trying to bring an old watch back to very high timekeeping standards can be difficult and expensive.
So personally, I have lower standards for vintage watches, but it is something of a sliding scale based on the age of the watch, the ease of obtaining replacement parts, and the sophistication of the movement. For example, I have higher expectations of a 1970s Rolex than a 1940s Eterna. For the latter, I would probably be satisfied if it is within 20s per day.
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