Also, I understand that Geoffrey Boothroyd convinced Fleming to switch from a Beretta to the Walther PPK. Do we know why he suggested that particular firearm?
While not Walther, after Dirty Harry the Smith and Wesson .44 Magnum saw a large increase in sales causing a price increase and trouble keeping it in stock. So it would only seem logical in this case also.
http://www.guns.com/2011/04/02/forty-years-of-dirty-harry-and-his-44-magnum/
This page has the correspondence between Boothroyd and Fleming:
http://dangerousminds.net/comments/the_story_behind_james_bond_and_his_weapon_of_choice