Great pictures! Are these New York skateboarders? These pics were probably taken during the clay wheel era. Before 1973 all wheels were clay before the invention of polyurathane. This information came from the documentary Dog Town and Z-Boys. The old clay wheels would lock up and the rider would go flying off his board!
Before polyurethane wheels. if you caught a rock, you were done. Plus, those boards have no tail which makes it almost impossible to pivot/change direction.
Check out the video released by the Canadian film board "The Devil's Toy".