“Wyoming is one of the wealthiest and most exclusive suburbs of Cincinnati. Ordinarily, bridge whist, ball dresses and all that sort of thing occupy the pretty heads of the two teams. Now they are poring over the rules as laid down in the Book of Spalding. By a process of comparison they have learned that a home run is equal to a no-trump make. A home run with three men on bases, is fully as valuable as a ‘no-trump’ grand slam on the rubber game.” Cincinnati Post, July 1, 1909. p5 [John Thorn]
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1929. “The game’s most remarkable feature was the stark fact that each manager, in his turn, picked a pinch-hitter who delivered a home run with the bases filled. One pinch-hitter thus producing what is known in baseball as a grand slam is enough to make a ball game momentous. When two managers inside of a half-inning thus score a tie on each other in picking grand-slam pinch-hitters the game becomes one for the ages” (William E. Brandt, The New York Times, May 27, p. 31).