Yes that pic is the hero before set dressing, as it came from Bapty.
The mystery disc was caused after a greeblie came off at some point during pre production. LIKELY, during the costume fitting when Harrison was getting acquainted with the prop and drawing it from the holster for practice. There are photos of this instance, but I only have a few, and none of the ones I have seen show enough detail to determine if the greeblie was ever present during the roll of film shot.
The lip you're seeing is glue residue. The entire lower was painted, minus the grips along with the mount. So what you're seeing is what looks to be where the glue discolored the steel underneath likely stripping the bluing... UNLESS... they sanding it down before paint for some reason. There's no way to know for sure, except for the fact that whatever cause the disc, had a hole in the center to allow the paint to make the dot.
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