I noticed that too, but couldn't say for sure. Same for the barrels: the bluing shows more age than the furniture.
I also have a hard time believing those screw heads left the factory without bluing of any kind.

I'm thinking that since no one has said they've seen a vintage British gun with engraving cut after bluing, the answer is that any gun like this should be assumed not to be in original condition. Whether that matters is a different discussion

Jim