Thanks TBM, you found the document while I was typing my reply. The bit that causes confusion is the wording "From April 2020 vehicles manufactured before 1 January 1980 are also able to display traditional ‘black and white’ number plates." People tend to take the date in the document as absolute, whereas INF104 is updated annually to reflect the 40-year exemption. As mentioned in the other thread, the rolling date for black/white number plates was apparently not intended but for some reason persists.
