This is a really good question actually. I cant find anything on what the factory used weirdly unless I've missed something. Whatever they used, it was good stuff and lasted. Im guessing it was some kind of good quality cellulose paint .
To the best of my knowledge, Volkswagen used synthetic acrylic paint for the 1968~79 VW Type 2s.
If you want to check whether yours has been painted with cellulose paint, just drip a little conventional polyglycol hydraulic brake fluid on it, which is reputed to be an effective paint-stripper for paint of this type.