Below are all the EOTech Holographic Optics that have a 1/3 Co-Witness. These optics come in either red dot or green dot reticle versions.

A 1/3 Co-Witness means that the dot of the red or green dot sights will sit about 1/3 higher than your iron sights. This allows you to have a more comfortable upright head position, instead of needing to dip your head downward for a closer cheek weld. 

Also if you are running your iron sights, they will not obstruct the field of view of the optic as much.

With a 1/3 co-witness the optics will sit slightly higher than an absolute co-witness. Also your iron sights will still be visible, but they will be in the lower 1/3 of the optic window.