These aluminium plates where fitted to those V12 cars converted by TWR.  This was only added when the engine efficiency kit. to show the change in specification from fitting the following:-
New exhaust with twin tail pipes (XJS) / single on XJ12
Large capacity air intakes
New filter elements
Improved spark plugs 
and modified engine settings.
 
These reprodutions have been manufactued by Jaguar's own supplier to a highest levels of quality. Strongest adhesive backing, although I would suggest some extra Araldite wouldn't be a bad idea.
 
Usually fitted to Bank A inlet manifold, although I have seen them also fitted to Bank B on occassions.