Mapae card case

Mapae (horse requisition tablet): 

In the Joseon Dynasty of Korea, a certificate that allowed officials to use the national language when going to the provinces for official business

In the late Joseon Dynasty, the Mapae was also a symbol of the secret royal inspector, a temporary official who secretly inspected public affairs on special orders from the king.

Nowadays, people also use it as a transportation card accessory by inserting a transportation card into it.

< Source >

https://www.culture.go.kr/index.do