The 2 channel revisions and old 3 channels (dual+triple) were referred to as "solo heads" and when the multi-watts debuted that got changed to the wattage of the particular amp (dual-100 watt head) and (triple-150 watt head)
There was a cross over period, so some of the newborns also say solo head, best way to tell the difference is the footswitch and the extra switch for wattage selection.