The square hole is for your winker horn!
Here is a little video so you can hear the annoying beauty!
If you do a Google image search for "cb750 turn signal buzzer" you'll be able to see what the part looks like.
The horn/buzzer has a rubber housing, with a square tab that fit's into the chrome hole you have there. Many CB750teers unplug the wires, but some went to the trouble of having them removed at the shop. I'm reconnecting mine, because people in my area just ignore "Don't Walk" signs and amble across the street without looking.
EDIT: BTW, love the bike. Gold is my favorite CB color.