When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
All non-metallic air-cooled cars are single stage through the 964 era. 993's seem to be variable by non-metallic color. I have seen original paint Black as early as 1995 in two stage (base coat/clear coat), but also seen single stage 1997 Guards Red cars, so I can't give a definitive date on this.
Metallic paints have been two stage since at least 1973 and possibly earlier. Here is the two stage sticker on my original paint 1973 Gold Metallic 911T:
Interesting side note, if you look at all early 90's late 80's gm and ford cars most have clear coat failure due to the manufacturer of the clear they used, early low voc formulas combined with not letting the first stage dry long enough caused adhesion problems
Also has to do with thermal expansion and flex and movement of paint