You have the time period correct with regard to helmet availability. Even if it wasn't completely dorky to wear a helmet riding a bicycle — where would you have

You have the time period correct with regard to helmet availability. Even if it wasn't completely dorky to wear a helmet riding a bicycle — where would you have even bought one in the 70's? Certainly not where you bought your high-rise handlebar, banana-seat bike. I recall no lawsuits being the cause of the adoption of helmets.