Bill,
my lamp is a 1930 BTH. It looks very similar. It has also a small hole between big wire hole and switch end.
May be it was used for an extra dip switch wire? There were wires though the hole. I do not know, is it an original hole or an modification.
I do use now only the big hole for the harness. I switch LOW - HIGH with the switch for the 2 lamps.
Michael