It is what button-collectors call a rivet-button. That type first appears in the very-late-1800s and is still being manufactured today. Rivet-buttons get used where an ordinary (sewn-on) button wouldn't be strong enough... such as pants-waist closure (blue jeans), and work clothes such as overalls/coveralls. Yours dates from the first half of the 20th Century. Scovill stopped making buttons in 1962.