LMG - M/60 Cannot Be Customized

UPDATE: Weapon can be customized once you unlock it on Level 20. The "Huntsman" is just a preset which cannot be adjusted unless you have the weapon unlocked.

As the title states. I've unlocked multiple attachments for the M/60 LMG, but it cannot be customized for some reason. 

1. Entered the "Loadout"
2. Selected M/60
3. Selected "Customize"
4. Can go through attachments, but unable to equip them onto the gun when double-clicking
5. Now a few matches later, I cannot even go into the customize screen for some reason. 

So at the moment the M/60 is unable to be customized (at least for me), and I am forced to play it as Stock.