Okay, this was bugging me so I went and pawed through my parts drawer of guitar and amp logos and found... the letter S. Since yours is a D, I'm guessing these were available to place the initial of your last name on... something. Like when I was a kid in the 1960s a lot of screen doors had aluminum ornamental bars that kept you from pushing through the screen, with a circle in the middle where you could buy your last initial and personalize your screen door. (mine isn't from a guitar or amp, just got stuck in there because it would have looked cool on an amp)