Ah, thanks for that Kev.
What looks like brown crud in the devices on some of the Medio Balboa dies looks to be residual gold (!) from when the dies were used to strike proof piedfort pieces in gold, which was unauthorized. Not that I would, but I suppose if I were to "restrict" unauthorized pieces from unconcealed dies that there would be a market?? Well, i'm not of a larcenous mind so that won't happen.