SO comes from sonderausfuhrungen which means 'special model' and was how VW designated that special models. For example SO1 was a westfalia's mobile shop, SO3 a police car or SO30 an ambulance. All these models where based on VW's Buses.

SO42 was ones of the last westfalia's camper conversions on a Split (T1) bus basis, before was SO22, SO23, SO33, SO34, SO35. SO44 and SO45 where contemporary to SO42 which was available between 1965 and 1967. 1967 was the las year of the Split(window), then came the Baywindow (T2), then the Wedge (T3), then...

The SO42 westfalia conversion is sure the most famous and the most ever copied all around.

Many other coachbuilders made similar conversions on panel or kombi basis but Westies where the only that could be ordered directly from the VW Factory.