According to Piggott's "Original Triumph", steel-wheeled sidescreen brake drums were painted black at the factory, while wire-wheeled cars could be painted black, aluminum, or silver. I used Dupli-Color Engine Enamel ("with ceramic"...) Aluminum on my TR3A (Alfins on the rear) and its worked really well. Finished smooth, cleans up well, and no subsequent issues with heat damaging the finish. Think I bought it at Autozone.
Sean