I cannot comment on 1968~71 VW Type 2s, but on my 1973 model-year VW "1600" Type 2, the chassis number is stamped into the rear engine-bay surround and on a rivetted aluminium plate behind the driver's seat, which also includes the front & rear axle load limits in kilogrammes and the Type number, which for the Kombi (i.e. Type 23) with VW factory-standard roof-aperture for an elevating roof, is Type 23-517.
It is these specified front & rear axle load limits (i.e. "plated weights") which define the minimum permissible load ratings / indeces for the tyres one fits.