This is an awesome topic. This is my favorite porno shot and the original poster was quite right to point out that it is a mainstay of 70's era porn. Unfortunately with the current demand for facials, the pull out fountain has become largely a relic of the past. I don't have clips, but 2 good examples would be ***** (1980) and Behind the Green Door (1972). In ***** during the orgy scene the first cumshot you see is the classic cowgirl pull out fountain. ONly better because there are 2 other chicks (I think Brook West is one of them) assisting. You don't see the dude's face, but you see him thrusting with the actresses ass riding him, he pulls out, cums mainly on his legs, and then one of the bystander girls puts his cock back into the rider's pussy. So classic!
IN Behind the Green Door during the famous trapeze scene, you see a slow motion reverse cowgirl pul out fountain. The cum gets caught up in Marilyn Chambers' pussy. Then you see the famous no hands pyrotechnic cumshot. The dude is masked and in a trapeze swing.