These Golden or "Excelsior Weapons" are Chapter Mastery Rewards. To get one, you must earn 5-star mastery on all missions (including the premium mission) of the corresponding chapter. The damage of these weapons scale according to the Agent's level.
The images for chapters 1-10 were screenshots taken when the Agent was level 50 with no Iso-8 bonus to Attack. The images for chapters 11-12 were screenshots taken when the Agent was level 139 with total Attack stat 695.