What does nvc inside of a ring mean?

I found a unique ring with the markings of 925 NVC 6...925(92.5%) means it is sterling silver and is the purest form of silver typically used in Jewelry making...The only thing I have been able to find on NVC is that there is a Distinctive handcrafted Jewelry designer named Nataliya V Collister...among other things she exclusively uses this type of silver....don't know what the "6" means yet.