Aprilaire #6508 Backflow Damper for model 1750/1770 dehumidifier. Normally closed, 8" round.
The model 1750/1770 can pull air from the primary living space or from the return of the HVAC system and places the dehumidified air into the supply above the air-conditioning coil in the main supply duct.
Please refer to this document for installation options.