The first image seems to have the shape of the baseball diamond.

The second image from a 1917 "The Literary Digest" magazine, is home plate, isn't it?

Did they use different PO Box numbers to track the number of replies to the ads?
A quick check confirms the conjecture. In a 1917 copy of the "The American Magazine" the home plate ad uses PO Box 437. The same ad in McClure's Magazine uses Box 407. Still using that ad in 1920 in Cosmopolitan, Box 401 was used.

Even better than an ad is something like the younger brother of a major league star and eventual Hall of Fame baseball player like Goose Goslin enrolls in the Wenonah Military Academy. Read Goose's story in the PDF below as it appeared in the Oakland Tribune (Oakland, California) · 20 Nov 1931, Fri · Page 35.

1911 WMA Baseball Team
1, Rogers; 2, Bilderback; 3, Holt; 4, Capt. Jones, Ath. Dir.; 5, King; 6, Heffler: 7, Jester; 8, Newell; 9, Walker; 10, Hipp.