The "Gilt" and "Imperial" and "Orange" are not back marks, but considered quality marks. They were common from the late 1790s and for the next 100 years. The quality marks such as "treble gilt", "superior quality", and orange colour" along with many other variations were designed to give some indication to the buyer that the buttons were of the highest standard, triple plated, and of superior quality. It was kind of like the more modern "new & improved".
I expect there is more information out there... maybe in a book.
That's a great looking button!