Stan,
Blacking the base plate, often along with the trigger plate, is the standard refurbishment for English boxlocks.
Whether the guns left the factory like that is a whole other question.
One of the true gunmakers would need to post on that.
I am just a 'restorer' but since the base plates are rarely case hardened in my experience, I suspect it was normal for new guns to have blacked base plates.
The action may be 'coin' finish now but I am almost certain it would have been colour hardened originally. In fact I think I can see a tiny remnant where the balls meet the bar.
