Good question Peter.
Frank the first player that came to mind for me was Jose Valdivielso too Frank, so odd.
I once heard that players give their teammates nicknames. It's kinda a right of passage. Now weather those nicknames make it to the general public, is another thing. Like Magic Johnson was called Buck by his teammates (cuz he helped them make bucks), but to the public, he was Magic.
On Koufax, i did recently see an old NY Post newspaper headline call him Koo. Any older NYers or LAers remember that?