Bicycle helmets! In that one set of frames up above, the dad is wearing a bicycle helmet. See, those didn't become popular until the '80s. During the '70s (which was my early childhood) we all rode around without them; me and my pack of friends would pretend to be Evel Kenevel too, doing stupid stuff on our bikes. Amazing to think of that now!
So yeah, it may not seem like it now, but seeing the dad wearing a helmet was a new thing back then. I think there may even be a Calvin & Hobbes in which they discussed helmets somehow.