Answered April 18th, 2019

Sandy Koufax strikes out 18 batters to establish a new NL record and tie Bob Feller's then MLB record for a nine-inning game.

Who was the first NL pitcher with 18 Ks in a game with fewer than 10 IP?
Interpreted as:
Who was the first NL pitcher with 18 or more Ks in a game with fewer than 10 IP?