Technician reported forgetting to install seat belt shackle attachment fitting causing seat to not have attachment points for seat belt.

Narrative

Installed seatback and hydro-lock per the CMM (Component Maintenance Manual) 25-62-82 for log-page XXXXXXX and XXXXXXY. Upon installation forgot to install the seatbelt shackle attachment fitting (P/N 1022492-125) causing seat to not have attachment points for seatbelt. During initial inspection of the seat; discovered various missing hardware and attachments which required researching and ordering parts. The seat had been worked on prior to arriving at ZZZ. Many parts were missing and not delivered on time during shift change. Time constraint was a factor as well due to scheduling for international flights. Towards the end of shift there was no manpower relief by oncoming shift; the resulting decision was to work overtime reluctantly to finish the completion of seat. Fatigue became onset during overtime hours and the cleaning crew were working within proximity of the work area that could have distracted line of thoughts. The seat was reassembled to the best of ability and appeared to be complete. Unfortunately; the shackle was not installed after reassembly because there were no spare parts remaining thinking the seat was complete.Suggested Resolution: Have hardware not misplaced and attached with seat or parts list of missing hardware damaged parts.