Re: puzzled by Screened bottom board design.
The best I can tell from your picture is what I bought about 5 years to be used for catching falling mites on sticky boards. They were never intended for ventilation. Thus the new SBB that is open on the bottom directly to the frames in the bottom chamber. It is designed for mite counts with the plastic cardboard insert or can be closed off with the insert leaving what ventilation you want. I leave mine wide open year around, but in Canada you might want to close it part way during the severist cold weather.
So much to learn, so little time!!