These body number plates were attached by the maker of the bodies just to keep track of their movement. Picture of these bodies arriving at the factory are out there. Bodies would be dollied up ready for the move into the factory. QC would check the body for dings and other defects before the bodies went to the paint shop. Bigger defects would mean that the body could lose its place in the queue for quite some time. Hence the mismatch to the chassis number. This is the same for Amazon and PV bodies. I think the practice started around 1960 as early PV's don't have the plate AFAIK. With the 1800 it might have started so that they could track the bodies for the UK produced Jensen production and then used for all other model bodies.