Sergeant Cooper!
When George tries to enlist in the army, the non-com officer at the desk hands a piece of paper to a soldier and says, "Take this to Sergeant Cooper."
Get it? Sergeant York/Gary Cooper! Another Warner Bros. film...from the year before! The WWI scene in "YDD" contains an in-joke about (and pays tribute to) the studio's most famous WWI film!
That's pretty cool, me being the first person in history to ever figure this out. Do I get a prize? I'll take it in 10's & 20's.