AN EMB-135 MECHANIC REPORTS OF A POSSIBLE INADVERTENT RE-INSTALLATION OF AN OXYGEN BOTTLE DUE TO BOTH BOTTLES STILL SERVICEABLE.

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OXYGEN BOTTLE (PART #176965-14) SERIAL #X; MAY HAVE INADVERTENTLY BEEN REPLACED BACK INTO ACFT X -- BOTTLE WAS SHOWING SERVICEABLE. AS BOTTLE WAS SECURED BACK ONTO ACFT; ACFT X BOTTLE AGAIN WAS CHKED FOR SERVICEABILITY; SHOWING SERVICEABLE ON THE GAUGE; THEREFORE; NO DISCREPANCIES WERE NOTED AT THAT TIME. ACFT X WAS LOCATED AT THE MAINT PAD. THE REPLACEMENT BOTTLE (PART #176965-14); SERIAL #Y; WAS COMING FROM THE HANGAR OFF OF ACFT Y. DURING THE RECEIVING PROCESS OF A PORTABLE OXYGEN BOTTLE; IT APPEARED THAT THE BOTTLE SENT TO THE VENDOR WAS THE SERVICEABLE ONE; POSSIBLY PART #176965-14; SERIAL #Y. MAINT RECORDS HAVE AGREED TO DO A SPECIAL REQUEST TO HAVE OXYGEN BOTTLE PART #176955-14; SERIAL #X; REMOVED IF INSTALLED; IF IT IS NOT THEN TO NOTE AND RECORD PART NUMBER AND SERIAL NUMBER OF BOTTLE INSTALLED. THIS EVENT MAY HAVE OCCURRED POSSIBLY DUE TO THE FACT THE NOT SO FAMILIARIZATION WITH THE PAPERWORK NEEDED; AND PROCESSED; FOR CANNIBALIZATION; OF PARTS OFF OF ACFT; THE TIME CONSTRAINTS APPLIED; THE BACK AND FORTH TO AND FROM THE HANGAR; CAUSING DISTR. MAY HAVE CAUSED 2 PARTS EXACTLY ALIKE BOTH SERVICEABLE AT THAT TIME; TO BE EXCHANGED. TO HAVE SOMEONE ASSIST IN A SITUATION WHERE MECH NOT SO FAMILIAR WITH SOMEONE WHO HAS MORE KNOWLEDGE AND EXPERIENCE IN THAT TYPE OF SITUATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THIS WAS A CABIN PORTABLE OXYGEN BOTTLE THE PILOT WROTE-UP FOR POSSIBLY BEING USED BY THE FLIGHT ATTENDANT FOR A PASSENGER. ALTHOUGH THE PRESSURE GAUGE INDICATED THE OXYGEN BOTTLE TO BE WITHIN SERVICEABLE LIMITS; HE DECIDED TO JUST REPLACE THE UNIT. HE HAD SEPARATED THE REMOVED BOTTLE FROM THE INCOMING BOTTLE THAT WAS 'ROBBED' (CANNIBALIZED) FROM ANOTHER ACFT IN ORDER NOT TO GET THE TWO BOTTLES MIXED UP. HOWEVER; ON THE PARTS TAG WHERE MAINT WRITES-IN THE SERIAL NUMBERS OF THE REMOVED UNIT AND THE REPLACEMENT UNIT; THE SERIAL NUMBER OF THE REPLACEMENT BOTTLE WAS ENTERED IN THE LOCATION ON THE TAG WHERE THE REMOVED BOTTLE SERIAL NUMBER SHOULD HAVE BEEN. WITH THE DISTRACTION OF DRIVING BACK AND FORTH FROM THE HANGAR AND THE LINE ACFT; BEING UNFAMILIAR WITH THE CANNIBALIZATION PAPERWORK ALONG WITH THE SERIAL NUMBER ALREADY ENTERED IN THE WRONG LOCATION; HE STARTED TO DOUBT WHICH BOTTLE HAD HE ACTUALLY REMOVED FROM THE ACFT. REPORTER ALSO STATED THE FLIGHT CREW OXYGEN BOTTLE IS BEHIND THE FO'S SEAT. PASSENGER OXYGEN IS SUPPLIED THROUGH OVERHEAD MASKS AND OXYGEN GENERATOR CANISTERS.