NavList:

I've been asked this question.  And I've asked it myself.  "What is a Class A sextant?  I've never found a satisfactory answer.

Here's a tidbit from random internet searching:

"This won't be too much use but perhaps it will stimulate some discussion. if memory serves me correctly (from the 1980s) sextants were designated by class. I think it was Class A, B or C.  The major ships had one high end sextant (Class A?) which was very modern with good optics, etc. The Navigator kept that one locked away and a good navigator wouldn't let anyone else touch it. The other sextants, which were kept in the chart table for general use by watch keepers, were a lower class. It seems to me that they were all British manufacture but by various manufacturers and with some minor differences in design and function. They all looked to date back to at least WW II and perhaps earlier."

Here's another one:
"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"

Here's a reference I could not open , maybe someone else can open it.
"A Critical Table for Sextant Corrections
journals.cambridge.org/abstract_S03734...
by JPG Worlledge - 1948
Jan 18, 2010 - The N.P.L. Class A Test, issued for instruments reading directly to 10"
"

I found a reference that states Kew Observatory charged 5 s[hillings] for a class A certificate, but did not mention the tolerance.

Brad