This real photo postcard dates from the World War I era and features a ballpark full of sailors watching a ballgame at the Guantanomo Bay Naval Base. According to the seller, the card came from a collection of a man who served on the USS Arkansas, so perhaps the picture depicts the ship's team on the field, or at least its crew in attendance.
A couple other things to note:
There's an officer standing on the rightfield line who appears to carrying a sidearm.
There are a few ballplayers among the sailors on the extreme left. Can we assume this is the bullpen?
Below are two versions of the card. The first is how it appears in the listing and the second is a cleaned-up image.