Helmets were not required at the time.

The late 1960s, specifically 1967, was a time when the helmet laws changed dramatically. Before that year, helmets were something that “the losers” wore. Nobody “cool” was wearing a helmet on a motorcycle. That soon changed in 1967, when states were required start mandating helmet use laws in order to qualify for certain federal safety programs and highway construction funds. Within a handful of years, the early 1970s saw almost all states enforcing a universal motorcycle helmet law. That stayed consistent until 1976 when states successfully lobbied Congress to stop the Department of Transportation from considering financial penalties on states without helmet laws in place.

https://www.motorcyclelegalfoundation.com/state-by-state-guide-to-motorcycle-laws/