904L is a special grade of stainless steel used by Rolex under the trademark name, Oystersteel.
Most watches use “surgical grade” 316L stainless steel, but Rolex uses the higher-cost low-carbon 904L alloy. This alloy contains nickel, molybdenum, and copper and is highly resistant to corrosion. The high chrome content allows it to be polished to a high sheen.