A B737-300 WAS DISPATCHED IN NON COMPLIANCE WITH THE L ENG HIGH PRESSURE TURBINE BORESCOPE PLUG NOT INSTALLED.

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ON DEC/FRI/02 IN ZZZ AT APPROX XA00 HRS; I ASSISTED ON A B737-300 DETAILED BORESCOPE OP/INSPECTION OF THE OUTER COMBUSTION CHAMBER ON BOTH ENGS. MYSELF AND 2 OTHER TECHNICIANS WERE WORKING TOGETHER TO REMOVE ALL PARTS REQUIRED SO THAT THE INSPECTION COULD BE COMPLETED. WHILE I WORKED ON THE #1 ENG; REMOVING THE FUEL NOZZLE BOLTS; ANOTHER TECHNICIAN REMOVED THE HPT BORESCOPE PLUG AND PLACED IT IN MY TOOL BAG. I WAS NOT INFORMED THAT THE BORESCOPE PLUG HAD BEEN PLACED IN MY TOOL BAG. AFTER THE INSPECTION WAS COMPLETE; I INSTALLED ALL OF THE PARTS THAT I HAD REMOVED FROM THE #1 ENG (FUEL NOZZLE BOLTS ON OUTBOARD SIDE) AND ASSISTED ANOTHER TECHNICIAN ON THE #2 ENG LEAVING 1 TECHNICIAN TO FINISH THE #1 ENG. WHEN I FINISHED WORKING ON THE #2 ENG; I WENT BACK TO THE #1 ENG TO FIND THAT THE COWLING WAS BEING CLOSED. I PICKED UP MY TOOLS AND CHKED THE GND DIRECTLY UNDER THE ENG FOR FOD AND ANY PARTS THAT MAY HAVE BEEN LEFT OUT. NOTHING WAS FOUND. THE FOLLOWING NIGHT AFTER COMPLETING A TIRE CHANGE ON ANOTHER ACFT; I FOUND THE BORESCOPE PLUG IN MY TOOL BAG THAT HAD BEEN PLACED THERE THE NIGHT BEFORE BY THE OTHER TECHNICIAN. I IMMEDIATELY NOTIFIED THE 2 TECHNICIANS THAT I HAD ASSISTED THE NIGHT BEFORE ON THE BORESCOPE OP AND WE IN TURN IMMEDIATELY NOTIFIED OUR SUPVRS. THE ACFT WAS FOUND TO BE IN ZZZ1 FOR A LAYOVER. THE MAINT PERSONNEL IN ZZZ1 WERE IMMEDIATELY MADE AWARE OF THE SIT. THEY VERIFIED THAT THE BORESCOPE PLUG HAD NOT BEEN INSTALLED AND THAT THERE WAS NO DAMAGE TO THE ACFT. THE BORESCOPE PLUG WAS SENT TO ZZZ1 AND INSTALLED BEFORE FURTHER FLT.