The ARMS 22M68 mount was chosen by Aimpoint as a standard with their M2. If going off the receiver, you need the full spacer. If you are going to use just an Aimpoint which mounts best for use at a foward position, then you only need the ARMS #40A2 rear sight. If you may want to go to an optic that has magnification and extends over the top of the folding sight, then get the low profile #40L. Both ARMS sights attach via a compact nut that doesn't become a knuckle buster, they also include a tube of lock tight so nothing can come lose. The ARMS products are the top notch in QC and are standard issue to US a allied militaries, and have been for many many years.
Good shootin, Jack