I believe the slatted racks with deep hive bases were used to improve
the ventilation in the hive. I built 6 deep hive bases with slotted
racks some years ago for production of comb honey. They have been used
since then along with regular hive bases for extracted honey. They
seem to do a little better than the regular ones but not enough to
justify making them.

If you are using the deep bases for ventilation you have to use the
slatted racks with them. If you don't the bees build comb down into
the bases making it awkward to reverse the supers or move combs
between hives. A modern replacement would be a deep base with a screen
board which would provide extra ventilation and also some control of
varroa mites.

Best regards,

Donald Aitken
Edmonton Alberta Canada