Which material was introduced for the first time in the Audemars Piguet Royal Oak collection in 1972?

On the eve of the 1972 Swiss Watch Show, known today as Baselworld, Audemars Piguet gave Genta a task to create a luxury sports watch. The next day Genta presented the idea and the Royal Oak was born. The watch was different from others on the market, it was made of stainless steel and an integrated bracelet.