Just like that. the disc was set into hinged clamshell and protected the underneath glass from breaking. The centre was left open so you could see the approximate angles of the hr/Min hands.
If the metal once had silver plating on it, it could have been once a part of a cheap watch or clock. Just a guess
WizFromOz
If the metal once had silver plating on it, it could have been once a part of a cheap watch or clock. Just a guess

WizFromOz