It's apparently a built in peep sight as described here (sorry for the ttag link)
https://www.thetruthaboutguns.com/sh...ed-mini-sight/
"...we suffer more in imagination than in reality." Seneca, probably.
The peep is obviously their solution with extend the enclosed potion of the optic far enough back to enclose the emitter while still moving the rear pane of glass forward to allow the mounting screws in the same place. Holosun managed to do it without such a compromise. It remains to be seen how intrusive shields solution is.