In Battlefield V, there is a chrome skin for the MG 42 that says it can be unlocked via a special assignment. I found this reddit thread that asks the same question, but an answer was not found.
What is the assignment, and how can it be unlocked?
I believe the chrome skin is awarded after completing the mastery assignment with the gun. Once you level up the MG42, it will unlock an assignment. Complete that and you get the skin for mastering the gun.
Since this is a special assignment, maybe it isn't in the game yet? It is also, not inconceivable, that there is a bug in the game preventing either the assignment from being unlocked or the skin simply not being awarded.
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.