I have found the whole subject of P-53 Enfield classes to be terribly interesting and confusing.

On the subject of ordering and accepting into the British service - there seems to be only two "Classes" "Complete to interchange" referred to the production of Enfield Locks and the London Armory and represented fully interchangeable components. "Contract" were those arms that while not fully interchangeable, were subject to strict guaging and proofs before acceptance and built under contract in London, Birmingham and Liege.

Actual "Classes" in British use were for 1st Class - suitable to issue for the regulars. 2nd Class - suitable for Volunteers and militia and 3rd Class - suitable for only drilling at regimental depots. Repairs to a 1st class arm would drop it to 2nd class and further damage or wear could get it to 3rd.

On what came to America - I have not seen that much on the use of acutal British classes.

Hope this is of interest. - Gene