Sorry to bump, but whenever I find new pieces of information about an item I picked up....I geek out.
I got in touch with Jim Leeke, who literally wrote the book on this game, and several others about war time baseball. He was excited to see the RPPC. He has seen the cropped image printed in the London Daily Mirror, but never a RPPC. A cropped version of this image ran in the Mirror newspaper in London on July 5th. The Imperial War Museum in the UK and the National Archives in the US have other photos of the July 4th game.
The Mirror did a centerfold pictorial on the game. Winston Churchill was also at the game.