The turned fluting on key stems is really only going to establish similarities between keys and makers, it’s never bombproof enough to narrow it down and identify a maker.
I have a small drawer full of HSS tool bits that I’ve ground to various point angles and radii for fluting keys- it wouldn’t be difficult to replicate that key using an old malleable antique blank with a similar bow, and carefully matching the shapes during turning.
i agree with oldlock in that any of the main makers would have put their mark on the bow, would be highly unusual and odd if they didn’t. Perhaps that ones an additional duplicate made sometime afterwards.
