Here's the flag. They used to put a loop in the cord. Did you ever notice?
6 seconds sound clip from the Stalag 17 movie soundboard.
You can hear this line at 01:50:24.435 in the Blu-ray version of the movie.
Quote context
[...]
- He spoke our lingo, so they sent him to spy school and fixed him up with phony dog tags.
- He's lying. He's trying to get himself off the hook. He said shut up.
- Okay, Herr Preisshoffer, let's have the mailbox.
- The one you took out from the corner of your bunk and put in this pocket.
- Let me show you how they did it. They did it by mail.
- Mail?
- Here's the flag. They used to put a loop in the cord. Did you ever notice?
- And here's the mailboxes. Hollow black queens. Cute, huh?
- They delivered the mail or picked it up when we were out of the barracks, like for appell.
[...]