"A Hat In The Ring"
Major Thorpe (writer and artist), Mort Leav (pencils), Al Bryant (inks)
Published in Jumbo Comics #33, Nov 1941 - 8 pages
At a cafe in Madrid, ZX-5 is the target of a poisoning in the hope that by eliminating him, the way is clear to attack Gibraltor. Following the assailants, he discovers a female British agent who has learned that the signal to attack the Rock will be a hat dropped into a bullring.
Here are the pages I have of it.