System Sensor SS24M - FireAlarms.tv - jjinc24/U8oL0's Fire Alarm Collection

That is an electromechanical horn on the rear of the strobe plate.

Category : Simplex

Since this is a ceiling-mount unit, it’s model number is probably different from 4903-9101, but I couldn’t figure out what...

Category : Amseco

That rubber band is holding the strobe to the unit since there are no screws keeping it secure.