Ok so using multiple wiki sites I stumbled across some unique armors than I'm struggling to find in-game.

Apocalypse Chestplate
Apocalypse Left Greave
Mark 4 Synth Chest Piece

Mark 4 Synth Left Leg

All wikis state these items are being sold at Level 4 Workshop Armor Merchant.

I have no idea what that means. The armor stores you can build from your workshop can only go up 3 tiers, the highest being the armor emporium.

My first thought is that these are faction rewards that the BOS and Institute vendors sell once you get a high enough rank but I'm not sure.

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