Did you find out what it is?
The Tempest F.6 also used it, its a tropical/desert aux-air filter, for dusty take-off engine protection.
They were trialed on the DAF Typhoons in North Africa, but were not available for the 2nd TAF units,
operating from dusty Normandy strips post D-day, which used a 'dome' intake cover, instead.
The Tempest F.6 utilized the whole 'chin' scoop for the radiator, & had the air intakes for engine & oil-cooler
set into the wing leading edges, but as trop-spec, was also factory fitted with the particular item in question.
Edit: Whoops, I see that the question was answered earlier, but if you wanted to know about
the Sabre engine black finish, its castings were pressure impregnated with a 'Bakelite' like thermoplastic,
as a preventative sealant against possible casting porosity - allowing fluid leaks.