Jimmy UK has it right. It's a small Drawer-knob.
For anybody who doesn't already know... it was installed by removing the slotted wide-top screw, then sliding the knob's shaft through a hole in the drawer's front , and then re-installing the wide-topped screw from the inside of the drawer.
Although some military collar-insignia had a "screw-stud back," there's no need for a slotted screw on insignia or buttons.