I have just discovered that this postcard was issued by the A.J. Schumann Co. out of Chicago. They did at least two other RPPCs of the West Side Grounds in 1908 as shown below as depicted in Ron Menchine's
A Picture Postcard History of Baseball. Some of their postcards had a stamp on back and some did not, a typical stamp is also shown. All of them had the distinctive handwriting on the front which was the hallmark of a Schumann postcard. A.J. Schumann was to Chicago RPPCs of the early 1900s as Thaddeus Wilkerson was to New York postcards during that same era. The work of each are remarkably similar and are real high spots in the RPPC collecting world. It appears that the Schumann PCs are much more rare than the Wilkerson PCs which are scarce themselves. A copy of a Wilkerson Polo Grounds RPPC like the one shown below recently sold at REA for $5,400.
Schumann 1.jpg
Schumann 2.jpg
schumann back stamp.jpg
wilkerson polo.jpg