I don’t know of any earlier topless shots, but she did a rear nude scene in a 1975 film called Rosebud. (Below. She’s the brunette on the right. She was 18 at the time.)
You may never have heard of this film, but it had a notable cast: Peter O’Toole, Richard Attenborough and Peter Lawford starred, and former NY mayor John Lindsay appeared in his only movie role. The five young naked actresses, who played kidnap victims, were not well known at the time, but at least two of them went on to lengthy careers. In addition to Cattrall, French screen legend Isabelle Huppert bared her buns on the deck of the yacht.
The trailer (below) is immensely entertaining if for no other reason than its utter incompetence. How could the Academy have passed up Mayor Lindsay for an Oscar nod?