applied to DVLA for the road tax class to be changed from PLG (Private Light Goods) to Historic and have the tax class on the V5C changed from PLG to Historic. INF104 also notes "vehicles constructed 40 or more years ago are exempt from VED/road tax. The 40 year exemption date rolls forward automatically each year on 1st April". But INF104 then adds "so from 1st April 2019 vehicles manufactured before 1st January 1979 will be eligible for exemption and permitted to fit "black & white" number plates".

Why does the VED annual exemption annual rollover also apply to number plates?
It does seem odd that permission to use "old style" black and white number plates rolls on in the same way as the eligibility to be a "Historic" vehicle for VED/road taxation exemption. You might have thought the black and white number plates would only be allowed on cars that originally had them, but that annual roll over is at present the correct position.

This appears to have been an oversight. When the Historic class was at the fixed date of 1st January 1973 it was correct, but when the eligibility for "Historic" status and VED exemption began to roll forward on an annual basis, the permission to use earlier number plates also continued to roll forward on the same basis. The UK authorities seemingly failed to pick up that it would have that effect, permitting all Historic class vehicles the use of black and white plates. At some time in the future the position may be corrected, but this would be unlikely to be retrospective, so vehicles that had become eligible to carry old style number plates would be