Echoing Oozeling

Drop: Irradiated Box of Assorted Parts
Drop Chance: 6 %

Drop: Box of Assorted Parts
Drop Chance: 2 %

From Atrius97 on Wowhead:

Irradiated Box of Assorted Parts is only obtainable once per day, per character, from completing the Hardmode version of the Reclamation Rig event, accessed by using the Supercollider weapon on Irradiated Elemental during the event turning them into Unstable Irradiated Elemental.

(Note: It is possible to activate hardmode WITHOUT turning all elementals unstable. Players are reporting hardmode activating having missed "1 or more elementals." It seems as long as most of the elementals are turned unstable, the hardmode version of the bosses, Unstable Irradiated Golem, will still spawn.)

Box of Assorted Parts drops after Reclamation Rig's Normal Mode.

Farming it additional times the same day on the same character who completed it will only reward Recycling Requisition, (or sometimes Strange Recycling Requisition), as usual for a second completion of the event. This resets with all other daily quests.