Major Thorpe (writer and artist), Witmer Williams (pencils and inks), Igor Shop (pencils and inks)
Published in Jumbo Comics #27, May 1941 - 8 pagesZX-5 is in a cafe at an inn in the Balkans when he observes a man keel over and die. The waiter pockets a note hidden in the man's sock while his female companion exits quietly. Intrigued, ZX-5 follows her and gets involved in recovering info on a treaty between several neutral countries joining together to stave off invasion.
Here are the pages I have of it.