What board is under my sink?

[Objektiv]

Under my old sink, there is this piece to insert into some tracks as a back panel/bottom "behind" the actual kitchen cabinet. It is 6mm thick and seems to be white-laminated on one side. A part is missing and this one is in the condition shown in the pictures, so I would like to complete and replace it. Which product should I look for?

 

[BirgitS]

There are masonite boards that are 6 mm thick, and masonite boards have often been used to construct cabinets.

 

[Objektiv]

There are masonite boards that are 6 mm thick, and masonite boards have often been used to build cabinets.

Yes, it could very well be masonite, but it looks like it's been painted from the factory. But maybe I'll just buy 6mm masonite and paint it myself.