A Line Mechanic reports he had to unbox and completely assemble an Aviox Passenger Oxygen unit as a replacement for a B777-200 cabin Aviox bottle that a Flight Attendant found with the Emergency Use decal missing. After departure; Mechanic realized he had not reset the bottle actuator pin correctly.

Narrative

While checking an interior lavatory write-up on an inbound flight; I was approached by Flight Attendant who showed me an Aviox Oxygen (O2) unit at 2H that had the Emergency Use decal missing. Not knowing if unit had been used I removed unit and ordered a new [Aviox] unit. Upon arrival the unit had to be unboxed and assembled completely.After reading the Maintenance Manual (MM) 35-30-99 I assembled the unit and reinstalled it in the aircraft. This was the first time I have had contact with one of these units. I made an entry into our Non-routine Maintenance computer and signed it off. The plane left the gate and I was taking care of the old Aviox unit and was examining it; and realized that I had not reset the actuator pin correctly. I did not want any issues further down the line so I immediately contacted my Lead and we contacted Maintenance Control Representative; Mr. 'X'; who was very helpful and we were able to straighten it out over the phone with ZZZ Maintenance as to exact location top or bottom unit in the Doghouse [compartment].