Medela Item #8207013 is a single white membrane with an offset mount. All Medela breast pumps except the Freestyle Breastpump require the use of white membranes.
It is important that the white membrane lie flat against the yellow valve. If the white membrane is getting old it may curl, or get a hole or split in it. Trying to use a membranes like this will decrease pump suction significantly.
The yellow valve and white membrane should be removed from the breastshield connector after each pump use. The white membrane should be removed from the yellow valve and both parts cleaned. If these parts are not cleaned milk will likely be trapped between them leaving the white membrane sticky. This will prevent the white membrane from moving properly during pumping and decrease effectiveness of the pump.
Note: This item is a single Medela white membrane. If you use new white membranes on a regular basis the six pack is a better value.