Eureka, I found it!    Go to the Grainger Catalog Page 2962, Part Number 1A853, Price $23.54.  This valve allows free flow in one
direction (Flaps Down) and an adjustable flow in the opposite direction (Flaps Up).  I slotted the valve stem to accommodate a
screw driver and put a large screw in the seat bottom right over the valve stem.  If I need to adjust the valve I can remove the
screw and expose the valve stem.  Any flap speed up or down is available.

Jack Hickham
Hilton Head Island
99.9% complete IVP
and still in denial