Dont believe everything you read on the internet, that’s not the original car, only one of the 3 still exist and that’s the white Briggs Cunningham car.
From the DVLA website, seems pretty simple to me. Time isn’t based on date of registration but on second time it is taxed.
https://www.gov.uk/vehicle-tax-rate-tables
There’s something in the back of my mind that says the Martini 155s actually had the Volvo/Renault/Peugeot V6 like you got in a Alpine or DeLorean rather than the Busso V6