Nice, but still not the complete answer, I think. My 1675 is gilt, and has gone tropical, but the black of the dial is as brown as my non-gilt 1665, and here's the odd part: the lettering is definitely gilt but it's a bright brassy gilt, not the deep rich orange/honey gilt like my 6536.
And take a close look, there are apparent "water drop" marks where it left "gilt droplets" on the brown background (this is why the dial was surrendered to the A.D.) It's almost like it leaked and the droplets affected the lacquer. But how would that reduce the opacity/tint of the background base paint? I definitely agree it would affect the lacquer's gloss finish.