Warren Spahn's Zipup Jersey

Warren Spahn was a superb pitcher. The all-time leader in pitching victories by a lefthander, Spahn's greatness holds up even under a more statistically inclined investigation. He never struck out a ton of batters, averaging only 4.4 SO/9 over the course of his career. But this unfortunate tendency to allow batters to make contact with his pitches didn't keep Spahn from compiling ten seasons in which he tallied at least 5 WAR, with a peak of 9.4 WAR in 1953. So yeah, obvious Hall of Famer.

The most surprising thing about Spahn, however, might just be his uniform. Take a look. There're no buttons on his shirt. The thing zips up.

This is crazy to me. I ran across a photo of Spahn in this zipup jersey over the weekend and I'm completely baffled by it.

I can just about picture Eddie Matthews repeatedly unzipping Spahn's shirt in the dugout. Baseball players love pranks.

Was this a standard practice in the early 1950s? Did the entire Boston Braves team wear these uniforms or did Spahn have some sort of aversion to buttons? Anyone have any more information on these shirts?