Last fall I bought a used "only one run" pair of Kastles with the rebranded Marker "K12" bindings. Like many of these that I have seen for sale online, the top of the "virtually new" heelpiece was a mottled brown discolored mess. Functionally it was fine, but it just doesn't sit well with me to have a pretty, expensive, white pair of skis with white bindings and a heel that looks like it was fished out of the septic tank.
Finally tonight it bugged me too much. I found that if I was very careful and slow with a smallish flat-blade screwdriver, I could pry up the clear plastic covering on top of the heelpiece body, without - shhhh - breaking off any of the three plastic tabs holding it in place. Having done that I quickly understood the origin of the problem. Two blobs of dark tan grease lurking in the top of the heelpiece, under the branded covering, had leached out under and over the wafer-thin mylar-esque sheet that hosts the logo, and also onto the bottom surface of the clear plastic covering. Several passes with an increasingly clean alcohol-soaked rag over all the offending pieces brought them 90% of the way back to looking like new. All snapped back (carefully) in place fine.
Shop rats, you no doubt know all this. Others, rest-assured this is an easy DIY fix.