Hmmmm, imports of foreign cars were subject to quotas in Spain until 1980 (Dictatorship + self-sufficient economy, you know the stuff). However in the 60s there was a significant US presence due to the strategic value of Spain in the cold war.
The US buyer was probably in the military since there were several US bases in the country. I doubt there was a Porsche dealer in Madrid (or Spain) in the early 70s, so the stamps were probably from a shop that did foreign cars for the lucky ones that could get past the quotas.
Besides foreign nationals, I bet only bullfighters and Julio Iglesias could afford a new 911 in the 70s
