What are these KAC parts?

[Wheres-Waldo]

Found two of these while sifting through my gun room.

The mating surface on top does not fit an RCO.

 

[Jackalope]

EDIT: pretty sure it’s for a PVS-4.

Weird. Kinda looks like it’s for a thermal or something. Similar to the Trijicon IR Hunter mounts. Looks too old though. No part number?

 

[Strykervet]

My vote is PVS4. It's a big dedicated optic, that much is obvious.