The Intel Dual Band Wireless-AC 8260 is available in 2 different board form factors: M.2 2230 and M.2 1216 (M.2 1216 is a soldered down version)
The M.2 2230 version has the standard E and A keys. As can be seen in the picture from Intel Dual Band Wireless-AC 8260 Product Brief.
We would like to mention, that even if a determined computer has the E or A key it does not mean that it will support this wireless adapter. We strongly advise you to contact the manufacturer of your computer to confirm which adapters are supported by your system.