1000 Mils = 1 Inch (Mils with 1x "l")
100 Mills = 100/1,000,000 of 1 Inch thick (Mills with 2x "l")*
*However Mils is an official measurement where as Mills is unofficial or 'slang measurement' for a smaller variation of that unit. Which means there is not even any recourse for you plated Silver or Gold if it is not even the full 10,000th of an Inch thick plating, you have really just got the weight of the base metal (Copper, Zinc)
100 Mills = 100/1,000,000 of 1 Inch thick (Mills with 2x "l")*
*However Mils is an official measurement where as Mills is unofficial or 'slang measurement' for a smaller variation of that unit. Which means there is not even any recourse for you plated Silver or Gold if it is not even the full 10,000th of an Inch thick plating, you have really just got the weight of the base metal (Copper, Zinc)