The .25 ACP is the 6.35mm in Euro terms.
Boothroyd meant to suggest the Centennial for Bond's main carry gun, with the S&W M-27 .357 for the "car gun". One can immediately see the logic in that.
Fleming got his wires crossed and went with the PPk and the Centennial. Read, "Dr. No".
By the way, the guy playing Boothroyd in the film version of "Dr. No" looked nothing like the real man.
Oh: Boothroyd later told me that after the S&W stainless Chief's Special (Model 60) debuted in 1965, it would be the obvious Bond gun. But Fleming had died by then.
Lone Star