In the short story The Case of the City Clerk, Professor Peterfield is a scientist or inventor. In the story, he had invented an advanced gun of some kind. Rather than selling the design to one country or another, he had decided to hand the plans voluntarily to "the League" in Geneva (probably a reference to the League of Nations which was headquartered in Geneva). Before this could be done, the professor was murdered, presumably by people who wanted the plans. The murderers failed to find them, however because of a fortunate mix up. Apparently a woman had given the professor a recipe for ham. The professor had put the plans for the gun in the fridge and the recipe for ham in his safe.
Lucas Bonnington, a friend of Parker Pyne, needed someone to courier the plans to Geneva. Parker Pyne thought he had someone suitable, a bored city clerk who wanted some adventure.