BG2GWD
Some were originally assigned to certain post offices, but that post office has since closed down. The mailbox remains intact and has been unattended for a long time.
Some of them are also self-made landscape mailboxes by businesses, but they have remained in existence due to a lack of regulation.
I also suggest trying to find a mailbox at a post office or branch, or looking for a public mailbox and sending a postcard to yourself to see if you can receive it. Then, you can decide whether or not to use it.