The back of this card says it all:
"Nervous fidgeting with baseball cap earned him his nickname. Began in majors with Cards, then was traded to Phils in 1947. At end of 1948 went to Chicago Cubs. Led NL in hitting while playing with more than one club during same season. In three World Series and on two All-Star teams."
No need to know on base percentage, slugging percentage, stolen bases or doubles. All you need to know is that Walker had one of the best nicknames in the game and that he was a pretty doggone good ballplayer.
Sometimes simple is better.