Just a reminder on an important clue for backmark time-dating, which I've mentioned previously here at TreasureNet.
At least for American buttons, a backmark's lettering-style (also called "font") can almost always tell you whether the button is pre-1866, or post-1866. I've studied all of the photos of backmarks in the McGuinn-&-Bazelon book on backmark-dating. Very nearly every pre-1866 Americanbackmark used "serif" letters instead of "plain" letters. Serifs are the little tabs or hooks on the end of a letter's arms. On Just-dig's button, the backmark's letters have no serifs ...they are just "plain" lettering. When you see "plain" lettering on an American backmark, the odds are literally 99% that it is a post-1866 backmark.
Here's a photo of four various fake slave-tags, showing plain letters and serifed letters.