Right away I knew the picture was no later than 1952 because Boston in the the NL. I was going to start with 1952 and work backwards.
Another very helpful clue is the listing of the Cubs and Reds starting pitchers. For the Cubs it was #20. In the 50's that number was worn by pitcher Dutch Leonard. I just went through his game logs at Baseballreference and it didn't take long.
The scoreboard shows the top of the fifth inning on July 7, 1951. The Cubs would go on to lose the game 8-6 and Leonard took the loss.
The attendance for the game was 11,626. The bleachers were packed but the rest of the grandstands would have been pretty empty.
The game was moving to the top of the fifth inning. Red's catcher Dixie Howell was coming to the plate. He would double as the Reds would go on to score two runs in the inning to take the lead for good.
Another oddity: Look at the listing of umpires and you'll see just three listed. There was no second base umpire. It wasn't until the following year, 1952, that four man crews were used in all MLB games.