HI; There is no known "Button Grading" Service anywhere. We use different methods to Grade buttons. First is the Type. Then the Rarity. After that is Condition. Then Comparison. The basic Button History goes :
One Piece Pewter & Tombac = Up to 1720 or so.
One Piece Buttons = Up to 1820.
Two Piece Buttons from 1820 - Present.
After 1820 buttons are dated by design and style of the backing.
Some of the early 2 Piece Buttons had a locking type of shank that allowed it to be used for either a coat or a dress made with thick materials. These were primarily used by women for dresses though ok.
After that a button is defined by its design. Especially Military ones. Then the ornate buttons.
Anyways, I did not see your button or I could at least date and ID it for you ok.
Goodv Luck. PEACE:RONB