it's the difference between an accessory signal unit [likely not GM manufacture] that clamps on the column and the factory optional signal unit that dwells in a cup that surrounds the column, even if no signal unit is present - that is, "built in" signal units were offered from 54 on, before that, accessory clamp on units were available [probably from the dealer only, not the factory]
and this should be in the Electrical forum, the topic is not specific to panel/burb body trucks
Bill
