Re-Stocked, Steel Blue Square Corozo/Tagua "Five Squares Flat Pillow", 9/16" # SK-514
Square button with four square holes. It's almost 1/8" thick and slightly raised - 'pillowed' in the center. Steel blue-gray color. 9/16". Sold per button.
Vegetable Ivory/Tagua/Corozo: Click here or here for wikipedia articles about this tropical palm tree nut. Tagua has been used for buttons since the 1880's, before plastic.