Thanks Graig. Retrosheet.org does indeed show that the
Highlanders were hosting Washington on September 5, 1908; however, the Giants were
away on Monday, September 7, 1908. The one photo does state on the reverse that the "Royal Giants" [of Boston, I believe] were playing the Philadelphia Giants, so if it is the Polo Grounds, then I'm not sure why these two teams would be noted on the back.
Max