Some generalizations about the placement of water-bottles on bicycles in the peloton:
Before the war, two bottles on the handlebar, none on the downtube. After the war, one on downtube, and by 1950 or 1951, only one bottle on handlebar. Anquetil, in all the pictures I've seen from 1957 and onwards, had only one bottle on downtube, none on the handlebar, even when most of his fellow riders did use a handlebar cage. Gradually through the '60's, handlebar-mounted bottle-cages disappeared. I think I saw a picture of Thevenet using one in 1970, but that was the exception. About '73 or '74, bikes began to have braze-ons on the downtube and seattube, as today.
About 1956 or 1957, riders stopped wrapping sew-ups over their shoulders, even on the long mountainous stages of the Tour. The reason, I assume, was better vehicle and roadside support. I also assume that this was the reason why one bottle on the downtube was enough for Anquetil and those who followed. I don't believe it has anything to do with a change in riding style, as Jan Heine suggested. If you're going to sprint at the end of a race, you toss aside your bottle, and how much does can an empty cage on the bars affect handling? Besides, sprinting style didn't change in the '60's. With a 52/13 high, you get low in the drops and spin, not sprint standing up, unless it's an uphill finish.