What a great question!
I had a day off on Friday, so I spent some time looking over my Topps cards from the '60s through the early '80s. I also went through the Post, Kelloggs, and Hostess sets, plus some oddballs from the same era.
I did find some abnormalities.
Though he was #9 during his time with the Cubs, Tim Blackwell's 1980 card shows him wearing helmet #52. No Cub player wore #52 between 1966 and 1982. I'm guessing the picture was taken at spring training, where there are all sorts of crazy numbers being used and he just grabbed any helmet for the picture
I found a few more abnormalities, though not the wrong number.
This 1976 card shows Champ Summers with black numbers instead of white.
Yosh wrote in Jose Cardenal's initials instead of his number. Of the 22 card that I have of Jose, this is the only one that shows him wearing a helmet. I wonder if he always had initials?
Nope, this picture from 1977 has a 1 on the helmet.
And then there is this one of Joe Wallis, from the 1978 SSPC set
Was Yosh playing some sort of a joke here?????????????????????