FAQ say :
"What type of Stainless Steel does Rolex use?
Most watch manufacturers use stainless steel 1.4435 or 316L. Rolex uses stainless steel 1.4439 or 904L which is slightly different from 1.4435. Steel 904L and 316L have the same grade of hardness. The level of nickel discharge with 904L is slightly higher, causing an increased chance of allergic reaction when worn by people sensitive to nickel. While technically 904L is more corrosion resistant, this is typically a concern is in industrial applications using steel in equipment for handling chlorides, sulfur dioxide gas or other toxic materials. Almost no watch manufacturers except Rolex consider it worth using the 904L steel, especially given that the price for 904L is approximately three times that of 316L. (answer by Ayjay)"