A B767-300 FWD CABIN ENTRY DOOR 1-LEFT ESCAPE SLIDE WAS REPLACED FOR TIME. AFTER SIX FLIGHTS; THE REPLACEMENT SLIDE WAS FOUND TO HAVE BEEN USED; NOT SERVICEABLE SLIDE AND REMOVED EARLIER; ALSO FOR TIME; FROM A DIFFERENT ACFT.

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BEGINNING OF SHIFT; I WAS ASSIGNED A SLIDE CHANGE AT DOOR 1L FOR TIME LIMIT. MYSELF AND ANOTHER MECH PICKED UP WHAT WE PRESUMED WAS A NEW SLIDE. WE INSTALLED THE SLIDE AND HAD THE INSTALLATION INSPECTED. EACH OF US THOUGHT THE OTHER HAD FILLED OUT THE PART TAG. IT TURNED OUT WE HAD INSTALLED A USED SLIDE REMOVED FOR TIME EARLIER IN THE DAY. WE ENTERED WORK PERFORMED AND SLIDE PART NUMBERS AND RELEASED THE ACFT. NEITHER ONE OF US VERIFIED THIS WAS AN AIRWORTHY SLIDE PRIOR TO INSTALLATION. ACFT FLEW 6 LEGS WITH AN OUT OF TIME SLIDE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATES THE EVENTS STARTED WHEN THEY WERE PULLED OFF OF ANOTHER ACFT THEY WERE WORKING TO REPLACE THE FWD ENTRY DOOR 1-LEFT ESCAPE SLIDE ON WHAT WAS SUPPOSED TO BE A 'HOT' ACFT; WITH DEPARTURE TIME APPROACHING. THEY PICKED-UP THE REPLACEMENT SLIDE IN THEIR STORES; PARTS STAGING AREA ALLOCATED FOR NEW; SERVICEABLE SLIDES. HOWEVER; THE PARTS EMPLOYEE WAS FAIRLY NEW AND NOT FAMILIAR WITH THE POSITIONING OF SERVICEABLE AND UNSERVICEABLE SLIDE CONTAINERS. REPORTER FURTHER STATED; THEY JUST ASSUMED THE SLIDE IN THE 'NORMAL' SERVICEABLE HOLDING AREA WAS A 'GOOD' SLIDE. JUST HAVING WHAT WAS ASSUMED TO BE A 'GOOD' SLIDE AVAILABLE WAS SURPRISING; BECAUSE OF THE CONSTANT SHORTAGE OF PARTS. ANY ACFT PULLED OUT OF SERVICE IS SURE TO HAVE PARTS 'ROBBED' OR CANNIBALIZED FOR SOME OTHER ACFT THAT NEEDS A PART. AFTER INSTALLING THE REPLACEMENT SLIDE; BOTH HE; HIS FELLOW MECHANIC AND THEIR INSPECTOR VERIFIED THE SLIDE PART # AND SERIAL # WAS ACCEPTED INTO THEIR COMPANY'S MAINT DATA COMPUTER SYSTEM. REPORTER ALSO NOTED; THIS REPLACEMENT ESCAPE SLIDE PART # AND SERIAL NUMBERS SHOULD NOT HAVE BEEN ACCEPTED INTO THEIR MAINT COMPUTER SYSTEM; IF IN FACT; IT HAD ALREADY BEEN REMOVED FROM ANOTHER ACFT FOR TIME; AND THAT WAS ACCORDING TO HIS COMPANY'S OWN PROCEDURES MANUAL. BUT; THE NUMBERS WERE ACCEPTED AND ALL THREE OF THEM BELIEVED THE INSTALLATION RECORD REQUIREMENT WAS SATISFIED. SINCE THIS INCIDENT; COMPANY CHANGES HAVE BEEN; AND ARE BEING IMPLEMENTED; TO ADDRESS THE SLIDE STAGING AREAS; THEIR MAINT COMPUTER DATA ENTRY ACCEPTANCE CRITERIA; EMPHASIS ON MECHANICS' USE OF PARTS TAGS AND VERIFYING SERVICEABLE SLIDES.