Terry, what about 667120…
Factory new with painted taillights, and indeed still that way all through the 1980s when I was very familiar with, and drove many many miles on what was then an incredibly original and excellent condition XK120 DHC - I am sure I can find photos somewhere, albeit probably not digital…
But as happens, car was eventually restored, and as happens, now has chromium plated Taillights - see pic after restoration…
So now lots of mods, which is a bit sad I think, given this was such a spectacular/original car.
But I suspect the original painted taillights are these days rarely seen, which is why it is impossible to define exactly when XK120s had painted taillights - but as per earlier comment, it was all about the 1951 Nickel prohibition Use order. And then if you delve into the 1951 No.1048 Legislation, it is most complex defining what goods are subject and what goods were exempt, and then how that impacted on the various manufacturers such as Lucas, who made a variety of components, some exempt others not exempt. About same time the Nickel Plated Generators (I would like to correctly say Dynamos) were replaced with black painted Generators…