CAMBRIDGE, Mass. — It was, if you will, the ultimate preppy convention. As Harvard University marked its 350th birthday last week, it seemed only fitting that the event turned into a celebration of all things prep. There were horn-rim glasses, chinos, button-down shirt collars, boxy Brooks Brothers suits, worn-out Topsiders and rumpled navy blue blazers as far as the eye could see. So much for the men of Harvard. For the women, it was a stampede of cardigan sweaters, pleated skirts and discreet strands of pearls, which made it hard to ignore the errant Chanel suit or two. Retailer Stanley Marcus (class of '25) reminisced about the knickers and pastel tweeds he wore as a freshman while other alumni compared crimson class ties, an essential Harvard accouterment.