".450 Davis"? Um, no, I don't think so.
Your pistol more likely needs .455 Revolver Mk.I and Mk.II (interchangeable), also called .455 Webley, .455 Colt, and .455 Enfield.
Fiocchi still makes it, labelled ".455 Webley (Mk.I)."
If you have had it checked out by that gunsmith, ask him if .45 ACP might not work in it, also. (It won't extract without the use of a punch, but it should be within your pistol's safety margin.)