Who knows mate, looks like hot oil blueing to me, which I’m sure they were capable back then, but I’m no expert. Look’s different in the flesh, a browny bluey black.
I did wonder if it was sent for refinishing along with all the antique shotguns while it was in the big house gun cabinet.
The white infill seems quite decayed under magnification, not sure what was used or how old it is.
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Also some surface inclusions but as these are cast parts I’d assume that’s not uncommon. All the edges seem to crisp for that to have been rust pitting that was brushed out?
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Like I said there's no way of knowing for sure, i can only dream!
Cheers,
Matt