Box: The United States had 48 states during WWII, it has 50 now. The number 49 is exactly between these two figures.
Combination: You must first get into box 66. To find the combination to that, line the card up to the picture of a field full of aircraft, the holes will reveal the letters “B” (on the tale of a plane) and “s” in Photo Courtesy of the Mohave Museum of History & Arts”. Use this to get into Box 66, where you will find a tool (black light) that lights up the letters F & X on the card in hand.