The officially certified chronometer COSC watches were certified 50+ years ago. Obviously they met strict standards. With the time that has passed, is it expected that these watches hold up to these standards even still?
Someone actually put this to the test a while back, sent a vintage Constellation at his cost for COSC chronometer testing and it passed
Officially certified means nothing at this point. If a fine watch was properly maintained, it can keep time at COSC standards even if it was never certified as a chronometer. The skill of the watchmaker had a lot to do with it. Sometimes it's just luck, also. Remember that while Rolex and Omega generally sent the chronometer movements out for testing, other brands like JLC, IWC, Audemars Piguet and Patek Philippe had their own standards which were more stringent than COSC. gatorcpa
Condition is everything for this question really. Just because it once had COSC in no way means it still will run to those sort of specs, and just because it never had COSC does not necessarily mean it won't. Cheers, Al