Phil – you posed me a question there. Well I looked through a couple of hundred shots of various smokebox`s and valve plates that I took yesterday at the Weald and Downland show. Burrells, Alchins, Garretts, Fowlers, Claytons, Dodmans, Fosters,McLarens, Aveling & Porter. With no exceptions all the lettering rings where brass.
Actually found an old wilco can of gold spray, thanks to Rays reminder. Sprayed a test piece which had been finely sanded and primed with black. It was quite promising. See what another coat can do in the morning. I believe a clear matt topcoat on that may be near the mark.