http://www.americanaldes.com/PDFFiles/CapeDamper.pdf

Cape BackDraft Damper~the ultimate one way air flow valve

EFI's Consumer Division Residential Catalogue | Tamarack Backdraft Dampers

It is supposedly a better and more efficient backdraft damper than the metal butterfly types you here flopping back and forth in bath fans and other uses.

Being as it was above the tub, it may have been for the bath exhaust fan?

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )