That pole looks just like the flagpole in front of an office building a block from here. I know where it is! It's a defunct military base.
Some soldiers would try to be indoors during colors so they wouldn't have to stop and stand there saluting. Evening colors could catch them by surprise because it happened at a different time every day. Those boxes were built for such emergencies. Beetle Bailey could jump in and close the door so he wouldn't have to salute. It would be a breach of etiquette for an E-1 to jump in on top of an E-6. So they're marked, 1-6.