Hey Dick, I was thinking the top slots would be for the mail, the panel/door below the front slot would be for the pick up schedule and the lower door was where the mail man would retrieve the mail.
I did not notice the bulb inside, if that is original to the unit and it came with ad glasses it is probably some type of call box for law enforcement. If it had ad glasses it would have had a way to hold them in, wish there were some pic's of the inside.
Very cool piece what ever it was used for.