It's the 1953 Bristol 404 'prototype' which has been fitted with this bizarre rear fin. I can't find any explanation for this, or any period images that suggest the car ever wore such a device in period. The car has been at some major concours events, and for sale with a big dealer, but the fin is never so much as mentioned, let alone explained, whenever the car is seen in public. Personally I think this is a spurious addition but I'm happy to be proved wrong! File under 'needs further investigation'...