The reg wouldn't be 55 UXB - unissued series until recent times when DVLA used numbers before UX* as "age-related" marks, but only with three numbers. Are you thinking of UXK 54 (Black 1954 Cabrio) or there was a scruffy split-window, originally light green, with a number something like ** BXP - I'll have to look through my old photos. Got a feeling it changed hands, lost its plate and got restored as a cal looker. I really hope I'm wrong!