The below is written by one of the best. If he says it, it's going to be correct. This will help you some on establishing age.
TheCannonballGuy
You're correct, the typical relic-hunter's name for your find is a flatbutton. More specifically, collectors of Historical buttons call it a "1-piece" brass flatbutton. (The shank/loop is not counted as a piece.) The first 1-piece brass flatbuttons with a backmark (maker/dealer's name or quality-level marking) were made in Britain. Your backmark saying "Superfine" is a quality-level, which was originally set up by the Button-Making Guilds in Britain. For example:
Fine Quality
Extra Quality
Superior Quality
Superfine
Double Gilt
Treble Gilt
The word "Plated" in a backmark always meant Silverplate -- Goldplate was Gilt
The first backmarked flatbuttons had the backmark written in raised lettering. They start showing up in Britain "about" 1790, perhaps a few years earlier. A backmark done in indented lettering seems to have begun about 1810 -- perhaps a few years earlier. So, your indented backmark gives us a starting point in time-dating your find. The ending-point would be about 1835, when plain-front flatbuttons fell out of favor with the public due to the advent of machinery which could mass-produce INEXPENSIVE hollow 2-piece brass buttons with an "ornate" design or emblem on the button's front.
Because you dug your flatbutton with a very simple one-word indented backmark in the US, my guess for its date-of-manufacture is the 1820s. That is when the infant American button-making industry first became capable of manufacturing enough metal buttons to meet the demand from the garment-making industry. I should mention, a factor in my guess/estimate is the fact that the supply of imported buttons from Britain got cut off by the War-of-1812... and afterward many patriotic Americans boycotted British-made goods for about 10 years after the war ended in 1815.
