"The Invisible Plans"
Major Thorpe (writer and artist), Mort Leav (pencils and inks)
Published in Jumbo Comics #44, Oct 1942 - 8 pages
ZX-5 has trained several Nazi spies from London to Moscow. One of them is the notorious spymaster, Baron von Krotz. When ZX-5 tries to fool him, though, he puts himself in their grasps. Later a female member of the cell helps steal vital new aircraft plans and seeks to sneak the plans out by drawing the diagrams on her arm using invisible ink.
Good Line: "In the spying business, the loser hangs!"
Here are the pages I have of it.