Vertical slatted racks?
Parallel, maybe? As opposed to those that run perpendicular to the frames? They go on top of the bottom board, between it and the first brood box. Gives the bees more room, and supposedly provides insulation that allows the queen to lay further down on the frames in cool weather.
Our bees seem to like them in the heat. Have them on two hives and they have hardly any bearding on the hottest days/nights when the other two seem to have all the bees on the outside. Not sure about the winter claims, but they did swarm very early last spring so maybe the claim that the queen can lay more in cool weather with them has some truth to it.
Here is Brushy Mountain's description:
"Slatted Racks help the colony with ventilation chores. Less comb gnawing, more brood nest and straight comb-pulling are attributed to this version of Dr. C.C. Miller's favorite invention. Our rack is placed over the bottom board and left on year round."