Hi bikerbob!
The despatch record says November 1955, so it is only 3 months after the model change. It was then registered early 1956, maybe has been standing at the dealer for some time, or the new owner waited for spring to come before using his brand new bike, who knows.
The frame definitely is 1954/55 spec, and fitting an Ariel hub at the rear would require enlarging the hole for the swing arm spindle and fitting the cable bracket on the rhs frame loop. Also, as I have mentioned before, the swing arm and rear chain guard for 1956/57 models are markedly different. So using Ariel hubs is not a simple swap job, and the guys at BSA for sure would have used the QD rear hub on such an old stock frame, most likely together with the 8inch half width hub front.