Billiard balls and playing cards. Belgium is world leader in both.

Edit: forgot to add pigeons. Belgian competition pigeons are the most expensive in the world.

Edit 2: Belfries. Clock-O-Matic is a Belgian company and world leader in the automation of Belfries. It’s not that hard because most Belfries are located in Belgium and France, if not all of them.

Edit 3: It never ends. Roller coaster wheels. Let that sink in. No their country produces more roller coaster wheels nor is as good as we arr at it.