I know I dont have an original Duetto-engine. When I got this engine I only looked for the 536-number and the right stamp in the front head. Anyway, I have this engine. The number is: 00536 S11264. Have you seen the S before? As far as I can find out it is an Zagato 1600 jr. engine. This is a 2nd series 536 (I think). See the pictures. Can you help me?