Ok, Found the reward location of the units.
Back: Complete "East Fortress [Apprentice]" Ep 5 quest.
Arm: Complete "Shadow in the city of stillness" EP5 quest.
Leg: Complete "The legendary savior" EP5 quest.
These are all combat missions, and are part of the main EP 5 story. The stuff in between rewards RWB4s and UWBs.