RJ55, I won't be too much help on this either but I can at least point you in the right direction... (n.b. Cannonballguy if he's around is usually pretty good on these IDs)
In addition to seeing the remnants of what appears to be raised lettering of "GILT" rather than indented into the button, if you look at the 6 o'clock position directly south of the shank I see what appears to be the remnants of a star, with that possibility there may have been a second star on the back as well that has eroded away along with other wording like "Rich colour" or "treble" etc. Either way just based on what I see and the other examples on the net, the use of the word "gilt" more or less was put on buttons from roughly 1790 through 1870s, with exceptions of course. This particular button if we can find maybe another star or make out other wording (which I know looks pretty difficult) could place this button in the earlier 1790s to 1840s range.
Hope this helps a bit.