Could be steel or cast iron.   A magnet can stick to it. 

Approximatey 5.5" x 3.5" x .75" 

Weighs 10.4 oz. 

A Rochester friend told me he remembered in the 1950's, his Dad pulling chains through a hole in the kitchen floor that he hung from this kind of plate.  The chains were attached to the furnace in the cellar and regulated the amount of air drafting into the furnace. 

If anybody would like to identify this differently, please do.