In the blog post the builder, Chappell Customs, mentions the ambiguity of the model year:
"The actual model year of this Enfield is a mystery: the original number plate indicated 1955, but no paperwork was filed prior to 1973. “The VIN numbers on old Enfields really don’t mean much, and there are no good records we can find to verify the year,” says Chris. So, to get the bike on the road, he went with 1973."