TWO MECHANICS AND A LEAD DESCRIBE THEIR INVOLVEMENT WITH A SEATBELT HARNESS MISSING FROM THE AFT G3 BULKHEAD INBOARD SIDE ON A DEHAVILLAND DHC-400 ACFT.

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I WAS WORKING ON ACFT X; IT WAS A WEEKEND MODIFICATION ACFT. WHEN WORKING IN THE AREA; I SAW THAT ONE OF THE FLT ATTENDANT SEATS WAS MISSING A SEATBELT. I TOLD THE PERSON THAT WAS WORKING THE JOB THAT THE SEAT NEEDS A BELT. I THOUGHT THAT IT WAS GOING TO GET TAKEN CARE OF BEFORE THE ACFT WENT OUT. WHEN THE ACFT CAME IN; THE SEAT DIDN'T HAVE A BELT; SO I THINK THE PERSON I WAS WORKING WITH DIDN'T THINK IT NEEDED ONE. SUPPLEMENTAL INFO FROM ACN 803455: DURING MY SHIFT PERFORMED AN AIRWORTHINESS WALKAROUND ON ACFT X AFTER THE WEEKEND MODIFICATIONS PERFORMED ON THE ACFT; SIGNED THE LOGBOOK BEFORE GOING HOME. AT SHIFT TURNOVER; MR E NOTIFIED MR G AND MYSELF THAT THE ONE SEATBELT WAS NOT INSTALLED AND THE OTHER DAMAGED AT THE AFT FLT ATTENDANT SEATS ATTACHED TO THE BULKHEAD BTWN CABIN AND AFT CARGO PIT; AND THAT GATE MAINT WAS INSTALLING NEW SEATBELTS AND THAT THE FLT WOULD BE DELAYED. I NOTED THAT MR B HAD SIGNED OFF THE REMOVAL AND INSTALLATION OF THE BULKHEAD ASSEMBLY. WHEN I FOUND OUT ABOUT THE MISSING SEATBELTS; I ASKED MR B ABOUT THE BELTS AND HE STATED THAT THERE WERE NO BELTS INSTALLED WHEN HE REMOVED THE SEAT ASSEMBLY AND DID NOT INSTALL ANY SEATBELT. DURING WALKAROUND; FORGOT TO INSPECT SEAT/SEATBELT ASSEMBLY (THE BULKHEAD -- SEAT ASSEMBLY WAS REMOVED FOR ACCESS TO UNDER FLOOR REPAIRS). MORE IN-DEPTH WALKAROUND ON ALL PARTS THAT HAD MAINT PERFORMED. SUPPLEMENTAL INFO FROM ACN 803478: THE EVENT OCCURRED BECAUSE I HAD A MISUNDERSTANDING ABOUT THE PRESENCE OF THIS MISSING SEATBELT. THE AFT INBOARD FLT ATTENDANTS SEAT HAD TO BE REMOVED TO REMOVE THE BULKHEAD. GRAVEYARD HAD REMOVED THE SEAT AND HAD NOT NOTIFIED ME OF A MISSING SEATBELT. THEY EITHER RETURNED IT TO STOCK; THREW IT AWAY; OR IT JUST WAS ALREADY NOT INSTALLED. THE AFT OUTBOARD FLT ATTENDANT'S SEAT STAYED UNTOUCHED. IF IT WAS INOPERABLE; IT WAS LIKE THAT FOR A WHILE BECAUSE I DID NOT TOUCH THAT. UPON INSTALLATION OF THE AFT G3 BULKHEAD; I INSTALLED THE INBOARD FLT ATTENDANT'S SEAT WITHOUT THE SEATBELT HARNESS -- THERE WAS NONE TO BE FOUND; I ASSUMED THAT IT DID NOT NEED TO BE INSTALLED SINCE IT DID NOT HAVE ONE ON THERE BEFORE. THE TASK FOR INSTALLING THE BULKHEAD TELLS YOU TO INSTALL THE FLT ATTENDANT'S SEAT WITH REFERENCE TO A DIFFERENT TASK -- THAT TASK DOES SAY TO INSTALL THE INERTIA REEL (SEATBELT HARNESS). I DID SEE THAT; ONCE AGAIN IT WAS MY MISTAKE IN THINKING THAT SINCE IT WAS ALREADY MISSING OR NOT INSTALLED ON THE ACFT; THAT IT WAS FOR ONE REASON OR ANOTHER NOT NEEDED TO BE PRESENT ON THE ACFT. THE TASK FOR INSTALLING THE G3 BULKHEAD TELLS YOU TO INSTALL A PIVOTING FLT ATTENDANT'S SEAT (IF APPLICABLE) WITH REF TO A DIFFERENT TASK; BUT THAT ACFT DID NOT HAVE A PIVOTING SEAT; SO IT WAS NOT APPLICABLE. NOWHERE IN THAT TASK DID IT HAVE A REF FOR INSTALLING OR REMOVING THE FLT ATTENDANT'S SEAT.