In this case I must defer to the T-Net relic-ID helpers who known more about "pre-20th-Century" Civilian button forms than I do. If presssed to at least post a guess, I'd say your button is from sometime in the first half of the 1800s.,, because early manufacturers of brass 1-piece buttons had persistent trouble in getting the shank (loop) to stay attached to a brass button's body WITHOUT making it even more difficult by having to put the shank in a narrow-&-deep depression.