It’s a movie and definitely not a Twilight Zone episode. Basic research turns up the names James Clavell and Howard Koch, with their screenplay based on a Frederick E. Smith novel. But Hollywood Script Shop lists Serling as the author of the 2nd draft of the script.

Bridget Bower manages the Rod Serling Archive at Ithaca College, which includes a number of unproduced scripts. She found this record in the archive:

M-9 The Violent Sky
“633 Squadron” [was the final name of the movie, produced in 1964] Second draft; 4-6-1959; carbon copy; 188 p. indicated.

What you saw is quite likely a genuine Serling script. As you probably know, credit for screenplays is a complex area that is governed by contracts and internal politics.