According to Wikipedia,
The gun was described in the book & movie script as a small, flat, .25 caliber Beretta which had the grips removed and could still be grabbed by holding the frame itself. That apparently only fit one description of one gun in the 1950s --it had to be the Beretta model 418.
But when they actually filmed this scene in Doctor No, where "M" inspects Bond's weapon, the actual gun they used as a movie prop was a slightly larger model. Apparently they couldn't come up with a Beretta model 418, or didn't want to put forth the effort just to film this one short scene.