BULLETIN MOS MSC SB 49-7857R1 APU - REPLACE/REWORK FIRST STAGE TURBINE ROTOR ASSEMBLY. PART NO. 3840303-1. TO FIRST STAGE TURBINE RECOMMENDED ROTOR ASSEMBLY. PART NO. 3840160-7. SERVICE BULLETIN MOS 20-DEC-2005 MSC SB 49-7860R1 APU - REPLACE FIRST STAGE TURBINE ROTOR ASSEMBLY. RECOMMENDED PART NO.