That  would be an "M" key which is 4 PCIe lanes. If you look at the service manual here, you can see the slot.
I'm looking for more complete compatibility information for the M.2 slot on the XPS 8930 because I'm looking to add an M.2 SSD to my system. I've used the documentation and dell's own compatible SSD information but I'm not able to find the key type of the M.2 connector. Is the connector type B, M, or both? Does the connector support x2 PCIe lanes or x4? Any help/thoughts on this would be very helpful. Thank you!
XPS 8930, M.2 SSD compatibility