Need Help removing Mailbox (Architectural Mailbox 7500) from post
Those are pop rivets.  They are not for removing.  

The product would have been sold with those permanent fasteners already installed.  The mailbox was installed on the post after those were attached, therefore they cannot be important to removing the mailbox from the post.  

You can remove them if you really want to by drilling them out, however this will prove to be a waste of time. 

Have you looked inside the mailbox proper?  Sometimes screws hide there.