Although there is no very-exact match for it in the chart, according to the button-dating chart below, originally posted by SteelHeadWill, your pewter button with wire loop embedded in a "mound," dates from about 1760 to 1790. Your button differs in lacking either a mold-seam or spin-marks on its back, and it has a brass wire loop instead of an iron loop. But it appears to have a lathe-spun design (multiple tiny concentric grooves) on its front... which might mean 1760 to 1785, only a 5-year difference in its time-period.