The "Solo Head" is not a Multi-Watt, it's a three channel Recto. It has no switchable wattage settings. The three channel Solo Heads (non-Multi Watt Rectos) were manufactured before the multi-watt series.
A Multi-Watt head rated for 100 watts is a Dual Rectifier. It has four power tubes and two rectifier tubes.
A Multi-Watt head rated for 150 watts is a Triple Rectifier. It has six power tubes and three rectifier tubes.
I don't know if there have been any significant changes to the multi-watt's design since the first ones were produced.