The 1948 Gorman "dogfight" with a UFO

https://sofrep.com/106917/the-time-a-wwii-fighter-pilot-intercepted-a-ufo-over-north-dakota-in-his-p-51-mustang/

I think you can disregard the date that the article notes when Captain Ruppelt coined the term UFO:

"Foo Fighters evolved into “flying saucers,” that is — until an officer in the U.S. Air Force named Edward Ruppelt coined the acronym U.F.O., short for Unidentified Flying Object, in 1556."

RR