VL-1504-A is the C-channel circular gasket which snaps onto the perimeter of the VL-1504-B "spoked" metal frame with riveted diaphragm.
Together, VL-1504-A and VL-1504-B make the VL-1504 valve assembly for the complete Vented Loop Cap Assembly (VL-1501) - which is the metal cap plus the complete valve assembly that goes inside it.
"Warning - This product contains chemicals, including lead, known to the State of California to cause cancer and birth defects or other reproductive harm. Wash hands after handling. For more info go to www.P65Warnings.ca.gov "
www.P65Warnings.ca.gov