The army adopted a button front shirt for the first time with the sweeping 1937 uniforms changes that made trousers standard in place of breeches. Although this shirt pattern was supereseded in late 1941, it was still made and issued throughout WWII.
HOW TO FIND YOUR SHIRT SIZE: Back in the Dark Ages, before the Internet, shirts came in dress shirt sizes, not S.M.L. The first number is your neck measurement in inches. The second number is the measurement from the center of your neck to your wrist, with your arm bent.