[bbpostcards]

This private mailing card showing Pittsburgh's Exposition Park in the background is the earliest card intended for mailing I know of which shows a major league stadium (left side, in the distance). The card is postmarked September 21, 1903, just 10 days before the start of the first World Series game in history between the Boston Americans and Pittsburgh Pirates on October 1 at Boston's Huntington Avenue Baseball Grounds.

My website (link below) has galleries of early baseball postal cards, unusual baseball postcards, and pre-1920 World Series related postcards if you're interested in seeing more.