In The Maltese Falcon, Sam Spade's partner, Miles Archer, gets blown down a hillside with a Webley Fosbery aimed by Bridgette O'Shaunessy. Spade, shown the big hog leg of a revolver found at the scene, says, "Webly Fosbury forty-five auto. They don't make 'em anymore."
I was wondering if that was a mistake or perhaps the Webley was chambered for the .45 ACP round. You've just answered my question. Neither. It wasn't .45 ACP, but it was a semi-auto and a revolver.
I don't know if anyone ever converted one to .45 ACP, but I suppose it could have happened. Lord knows there are enough of the regular ones floating around.