G'day Mark,
Between all of us, and for our purposes, it seems to me it would best be described as a '55 (personal opinion only).
For the purposes of advertising for sale and registration in NSW it would be a '54
In NSW and unless they have changed the rules Vic, not sure about the rest, motor vehicles including bikes are recorded on build date not compliance, and in the case of older machinery date of first registration.
Most model seasons for cars are based on sales pre and post New Year as well as major annual motor shows, it gives (gave pre GFC) the importers a chance to display their new wares at shows while running out old models so that they weren't competing with newer versions with younger build dates after the change of year.