LUKE,
I think you are correct.
There was/is no need for a label for the defroster to be on the 47/48 "deluxe" heater box, because the defroster-flap on the box was/is controlled by a cable/knob mounted under the dash to the left of the steering column. For that type heater, the defroster label was on the bracket for the cable/knob mounted under the dash to the left of the steering column.
A "defroster" label ON any GM heater box was only needed/used when the defroster flap ON the heater box was operated/selected by a human hand moving the defroster lever/flap ON the heater box.
I hope this does not cause further confusion. Good luck.
