Sorry, you got that the wrong way round.
The current legal proof of marriage is a Marriage Certificate and it’s the only document that suffices. If it was different in 1964, there’ll be a HI statute dated between 1964 and now that defines the change - that’s how legal systems work. If there isn’t, then it hasn’t changed. Can you cite any such statute?
As noted above, people can and do get divorced who have never been married. That is a fact. The exact circumstances that result in a divorce-in-the-absence-legal-marriage apply to the Stanley-Ann divorce case. This being so, it cannot be maintained that proof of marriage was a necessity for divorce.