The Hunter's Seal tooltip indicates that you can exchange 5 for 1x Grade 3 Weapon or Armor Reform Stone at a NPC named Finlin in Tarif:

The Grade 3 Weapon and Armor Reform Stones state that you can obtain it through refinery or by exchanging 5x Hunter's Seals for 1 Grade 3 Weapon or Armor Reform Stone at the Jeweler of Tarif (Thyshelle Arms).

The Hunter's Seal states <Tarif - Finlin> while it's supposed to be <Tarif - Thyshelle Arms>