Stainless Steel Audemars Piguet Royal Oak Selfwinding Chronograph 41MM Blue Dial 26331ST. Model – Audemars Piguet Royal Oak Selfwinding Chronograph. Reference Number – 26331ST. Bezel – Stainless Steel. Dial – Blue Dial. Case Size – 41mm. Crystal – Scratch-resistant Sapphire Crystal. Bracelet – Stainless Steel Bracelet. Water Resistant – 50m.