As I was led to understand and its supported in many publications and on line....
From 68-72 VW used the Sunroof model as the body that was given to Westfalia to covert into campers (The 73 model was a transition year and just had a hole cut in the roof which meant more headroom because there was no sheet metal to hide the sunroof). In 74 when the poptop roof changed direction a special body was created and the roof was braced above to reduce flex in the body.
To increase the structural ridigity of the earlier models, VW welded a bellypan to the bottom of the frame.
215 703 715 C underfloor plate, center used on models 215, 216, 225, 228, 235, 241, 244, M60, M520. This means:
- double sliding door buses (model 215, 216, 235 or M520)
- buses with a sunroof (model 225, 228, 241, 244)
- buses with Eberspaecher BA6 (M60)
Some Westfalia's have a bolted on belly pan but it has no strength and it not to be confused with the earlier version. It will flex under it's own weight and is mainly to keep the BA6 gas heater safe from road debris.
The English Dormobile buses (and possible other conversions) usually had M191 on their m-plate which means the same thing. These were regular buses with the extra belly pan to give them more strength, so afterwards that British camper conversion company could install a pop-up roof.
The welded part is 215 703 715 C and the bolted part is 215 703 715.
If a Westfalia was equipped with an air conditioner then the belly pan was not installed because there was no BA6.