On This picture you Can see its just bare metal
Too bad you don't have some Evapo rust to soak it in.
Did check through my pictures and don't have a ton of them to offer and like yours they are old and well worn by the elements. Best we can do is to compare other surfaces in the same picture for comparison. Its interesting that if the finish is different on single exhaust plates it suggests that they may have had different suppliers for the dual and the single exhaust styles and the finish is a change from earlier years.
Of course you always can have that possibility that some of the returned plates make their way back into the system but the single exhaust style didn't cause the objectionable noises and rattles - the reason for removing them.
Here are so of the better pictures to aid the discussion unfortunately the matting surface would be more telling - that and the frame there