Way back in the 1980s in the clubmans class racing there was a single cylinder racing 500 class, which was fine except all kinds of bikes came to the fore, Hagon Hondas, and even 2 stroke 500 from the moto -cross bikes- they started to blow everyone away.
If you had an old British 500 single it was no class for you any more, as the more modern bikes came to the track costing half the price new, to a secondhand Norton Manx.
Sadly that is the way it went until the Classic racing club became more popular.