I'm not sure what other classification systems are in use around the world, but British or former-British colonies would likely use the Brit classifications, which I believe were set by the Royal Observatory - Class A means total errors less than 30 seconds and Class B less than 3 minutes(iirc). More important than the classification, would be the date of the last
inspection - a
sextant should have an error chart with it; if that is missing or dated, then you would have to measure it yourself or get it done, though I don't know where you would get that done.