These flat buttons were used for German uniforms during The First and Second world wars. They were sewn on camouflage blouses, shirts, pants (both cloth and HBT) and overalls of all kinds. In general this button is a classic, it can be used on any clothing.
Diameter 17 mm. Material: steel with zinc coating of the finished product. Original buttons, by the way, were rarely zincated. By the time the buttons started to be made of steel (1940), it was not possible to use expensive zinc for finishing such minor items as buttons. So they just painted buttons in gray. We zincate the buttons because it provides adequate consumer and storage properties. For full compliance, the owner can paint them gray by himself. Particularly meticulous reenactors may even remove zinc using chemicals and then paint. To the case, to wash a button without the zinc coating along with clothes in the machine is a bad idea... And you need to remember that the clothes on front did't live long (as well as soldiers), so rusty buttons were not a problem there. In contrast to the reenactment, where you use your clothes for many years. Our zincated button does't deteriorate from washing!
Four-hole buttons were used usually on clothes, when three-hole mostly for equipment.