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.