The Clue of the Gold Coin (UNABRIDGED‪)‬

"Vicki Barr is a popular mystery series for girls published by Grosset & Dunlap from 1947 to 1964. This book is volume 12 of The Vicki Barr Air Stewardess Series.

When a shipment of priceless gold doubloons being transported on her flight from New York to Tampa goes missing, Federal Airline's stewardess Vicki Barr is unwittingly dragged into a sinister conspiracy spanning two countries. Vicki faces a determined group of criminals willing to go to any lengths to stop her from discovering their secret. It is up to her to save the Gasperilla Pirate festival, rescue a trapped innocent old man and a young friend she helped in getting the very job that would risk his life and career. With the help of the FBI, Vicki takes on a ruthless gang of villains who have disappeared with the ancient gold and left no trace the police can find. (Summary by amitsharma)"