Saw these two on Kottke, which had found them on the Smithsonian Museum site. They say:
Plastic American Express credit card, 1963. The first plastic credit cards appeared in the early 1960s. By this time the old booklets were no longer very useful. Credit cards were beginning to be accepted by an increasing number of vendors making the booklets too cumbersome and negating their purpose. Plastic enabled vendors to use the cards in charge plate machines, making the recording of transactions easier and more accurate.