Hi wacky!
It definitely is 1954/55 spec. It has the mounting and stop for the Goldie-type rear brake lever, 1956 brought the Ariel hubs with cross-over linkage, and an additional cable bracket on the rhs rear frame loop, yours doesn't have that (due to lhs rod operation on the rear brake).
So half width front (8inch) and QD rear are correct.
I can't see details of your swing arm and rear chain cover, those changed for 1956 as well. The chain cover for 1956 onwards was shorter at the front (and the front part of the fully enclosed chain guard was fitted over the gearbox sprocket), and the attachment points for the chain cover and the brake torque arm on the swing arm are different. Looks correct on the last photo you posted though.
edit: speaking of it, the inner primary chaincase seems to be the later type, since it has the mounting hole for the afore mentioned front chaincase part (pic no. 3)
No need to run and search for a new one though, that's the only difference afaik.
Your bike being manufactured in late 1955 could also have been a mixture of parts from new. The 1956 model year would have started in August 1955 already, but BSA would also use up old stock of parts and use whatever is at hand.
Cheers, Markus