MG 42 Chrome customization special assignment unlock
This was a bug, and it no longer exists in the game as of June of 2019. The chrome skin for the MG 42 no longer says there is a special assignment; it is only obtainable with company coin.
The Urban Blue skin is unlocked through leveling up the gun, the Mint skin is unlocked by completing the MG 42 Proficiency assignment series, and the Gold Plated skin is unlocked by completing the MG 42 Mastery assignment series.