The Retro Field Uniform is a unique World War 2 themed outfit in The Division 2 that can be unlocked by completing the Navy Hill Transmission Side Mission.
You will most likely stumble upon this through your campaign walkthrough anyway, but in case you haven’t done so, it’s worth to give it a try to get this cool looking apparel.
Head over to the Truman Safe House in the Foggy Bottom District and move the place marked on the map. A ladder will lead you down into the Underground to a female mission giver called Jane Applegate. Speak to her, and follow the GPS.
A small riddle consisting of a painting you need to interact with and remove from the wall when going up the stairs needs to be solved, but that’s the trickiest part on this mission.
You will be rewarded with the Retro Field Uniform, and secret intel for another Side Mission called the G. Phillips Protocol where more loot is waiting for you.