if you can prove the machine was manufactured pre 73 you will get historic , and free tax , eg your car bike may carry a reg in the year 73 and still be historic , there are plenty of people who have used this , the year of registation is not the final word
heres the wording for the tax and mot exemptions
How we identify a vehicle as ‘historic’
You may be able to tax your vehicle in the ‘historic vehicle’ tax class if it was built before 1 January 1973 and is one of the qualifying vehicles listed below.
As part of the Reducing Regulation agenda and the desire to remove unnecessary burdens on society, the Government is proposing to exempt pre-1960 manufactured vehicles from statutory MOT test, as allowed under Article 4(2) of EU Directive 2009/40/EC, and bring the age of vehicles requiring the statutory MOT test in line with The Goods Vehicles (Plating and Testing) Regulations 1988.
hope that helps