Guys, I think these pictures are actually from the 1912 World Series. Somebody contacted me to say that he used to have a bunch of photos taken at the 1912 World Series, and these have the same look and feel. More important is something I noticed when looking at the grandstand picture on my desktop computer. At the far right edge of the picture, just to the right of the end of the grandstand, is a sign with "NEW YORK" on the top and "BOSTON" on the bottom -- presumably a scoreboard. It should be easier to see in this blowup:
Now compare that to this picture of the Polo Grounds' first-base grandstand before game 1 of the 1912 World Series, taken from Wikipedia:
https://en.wikipedia.org/wiki/1912_W...lo_Grounds.jpg
Notice at the far left, just to the left of the end of the grandstand, is an identical sign with "NEW YORK" on top and "BOSTON" on the bottom. Also note the white boxes in the first few rows of the stands, from which the term "box seats" comes. In my grandstand photo, shown again below for reference, you can see the same kind of white boxes in the first few rows of the third-base grandstand. I'm not sure whether these boxes were always there at the Polo Grounds or whether they were installed especially for the Series.
In theory, this could be a regular-season game between the Giants and the Boston Nationals/Doves/Braves. What makes that unlikely is the size of the crowd. My picture and the Wikipedia picture show different parts of the grandstand, but the crowd in each looks the same -- the lower deck completely full, the upper deck mostly full but with some empty spots here and there. The outfield pictures show that the bleachers were also pretty full, except for some empty spots way out in center field. Now, each of the three World Series games played at the Polo Grounds in 1912 had an attendance of more than 30,000, close to the park's capacity at the time. According to baseball-reference.com, of all the Giants' regular-season home games against Boston in 1912, the highest attendance was 15,000, for a doubleheader on September 28. The crowd in these photos of mine look like way more than half capacity. So, it looks to me like a 1912 World Series game.
The Giants' starting pitchers for the three games at the Polo Grounds were Jeff Tesreau in games 1 and 4, and Rube Marquard in game 6. If the mystery player walking in from the outfield is the starting pitcher after warming up, and he's a lefty, then it's presumably Marquard. But those are some big assumptions that I don't think we can necessarily make.