What is it ?

[nickk]

Found in a wall loose,on a building I was working on Monday.

 

[gaz1]

chimney chute block plate stops drafts coming down the chimney

 

[MoreWellie]

remains of a broken baffle from an oil boiler

 

[slim_boy_fat]

chimney chute block plate stops drafts coming down the chimney

A bit 'heavy duty' for that?

 

[Other Daniel]

A book-end ?