The original Flying Tigers was a group of civilian mercenaries officially known as the American Volunteer Group (AVG). Most were American military pilots who had resigned their commissions to join AVG. The organization was dissolved when the U. S. entered the war but many who had flown with AVG were inducted into the U. S. Fourteenth Air Force. 14th AF adopted the nickname "Flying Tigers" and were thus known throughout the remainder of the war.
For info on the 19th Liaison Squadron see Message #3118, this forum.