Robert Broderick has been quietly scanning ads from bike rags, and sent me some of the early helmet images he has:
first (or early) ad, for production helmets available:
MSR helmet: June 1974; Bike World. Bell: Nov. 1974; Bike World. CCM (hockey style, not good, Feb 05 Skid-Lid Travesty, Oct 1975, Bicycling Bailen: June 1979, Bicyling (although I think they came out several years before, I can't be sure of that. Small ad.
Thanks, Robert!
Certainly, one can wear a real helmet and ride (some) on-topic pre-1984 bikes with a clear conscience about being correct. But, before Eddie Alpert shouts at me, for bunches of reasons related to uncertainty about how plastics age, I don't recommend it. Except very rarely (like Cirque, when I will go out with an early Bell for the fun of it), I always ride with modern helmets.
harvey sachs
mcLean VA