I believe it is just part of the "quality mark" If it were to say "extra gilt" double, triple etc.. It would be the amount of gold applied to the button in this process------------------------------------------------------------
The first wonderfully sparkly gilt buttons were made in Birmingham between 1797 and 1800. They were elegant, yet affordable. People took to them immediately, and they became the height of fashion. Even the Americans began making gilt buttons by 1810, after stealing the gilding secret from the British!
Actually, the process was a fairly simple one. Five grains of gold per gross (144) of buttons was added to a mixture of mercury, then brushed on the brass buttons, which were then cooked in a furnace. Buttons could be double gilt, triple gilt, and so on, depending on the number of grains or number of brushings used.