Memory does strange things.  I was really sure that the year I saw it was 1964 because I had just arrived in California.  So yesterday, I went to the San Francisco public library and got the Chronicle on microfilm for July-Sept 1964 and went through the movie listings day by day.

The Clay was showing Moderato Cantabile (Jean Moreau) and The Five Day Lover (Jean Seberg).  

So I assume you must be right. Although I am still puzzled.  I know there was a review by Brendan Gill in the New Yorker and I will try to find it if some library still has archive copies from that era.

Did you see L'immorale?  Anything you remember about it?