Hey guys, I just wanted to jump in to assist!
Everyone is correct in stating that you can use a Mil-Spec geometry cam pin in our Geissele uppers. The notch in the upper is designed to function with our unidirectional cam pin, but it would be completely functional with a Mil-Spec one. Our cam pin has a raised edge to assist in uniform wear and power through carbon, this requires a relief cut. With that being said, its design is still "Mil-Spec" in nature so you can use a traditional cam pin as well. It just wouldn't take advantage of that feature.
Like others said, you cannot use the unidirectional cam pin in a non-Geissele upper. Since that relief cut is not present, the cam pin will make contact with the inside of the upper and not permit the bolt to lock into battery.