Taz42o, here's the scan from McGuinn-&-Bazelon's book on backmarks I promised to post. That book is a far more reliable info-source for backmark-dating than any website. The book only costs about $25 ...and it will pay for itself multiple times for anybody who is thinking about buying or selling (or metal-detecting for) Historical buttons.
By the way, this scan is made and posted with the permission of my longtime friend Wliiam F. McGuinn, one of the book's authors, who said I could post an occasional scan from the book if I only do it for Educational posts.
Although the Scovill button-making company was founded in 1811, this scan shows only the various Scovill backmarks from 1860 to 1920. (Earlier backmarks are shown on the prior page in the book.) IMPORTANT NOTE: as I said in my reply to Plymouthian12 ...the presence or absence of "stars" (or other "punctuation-symbols" in the backmark), a circular solid line around the loop, a circle of dashes, a circle of dots, and even the size of the lettering are VERY important clues in accurately dating a button's backmark. At first glance, some of these backmarks seem identical -- but look for the small differences. They can matter a lot.
(By the way, for anybody here who doesn't already know... left-click on the posted photo to see a larger version of it.)