The 54 models had different cylinder heads. If you look at a picture of one closely, you can see this. They still had a seperate oil filter from the oil pump/gear cover. They hadn't started using alternators yet in 54 so they were using a Lucas distributor/dyno set up. I think they were still using a smaller carburetor but it was still an Amal Monoblock. The small journal crankshaft were used up to 56. The con rods were not using bearing shells in 54. The crank changes made it possible to use replacement half shells.
Royal Enfield fielded a lot of newer changes on the USA market first so in many cases the Indian bikes recieved these changes before the home market UK ones did as far as production is concerned. The USA bikes tended to see some rough service here so RE was trying to beef up a twin that was originally designed as a 500cc so it needed a lot of beefing up as a 700cc (692cc) especially under harder service conditions.