These are models for three hollowed button molds/forms for fabric-covered buttons, such as those commonly use in 18th-century fashion. Unlike wood forms, these do not have a flat back, which allows bulky fabrics to turn inside the mold, reducing overall bulk. These are not buttons themselves, but rather the forms around which you can stretch fabric to create a covered button. In one of the photos you can see the difference in bulk between a traditional button form and one of these when using a woolen broadcloth. There are three sizes. The 18 mm size will give a finished button about ¾". The 24 mm will yield ~1" buttons, and the 28 mm will provide a finished size of about 1-1/8". 

I found these print best with the convex side facing down (like a dish) using supports and a raft. If you print them face up, removing the supports can be challenging.

The center hole can be use to center and pin fabric while covering the button, but is otherwise not needed.