Not sure, but I think the mirroring Mark is referring to is silver migration, where the metallic silver migrates to the surface and forms a reflective metallic sheen that may have various colors common to tarnished silver. I don't think it's that though, as that shows up in reflected light, and in this case, the negative is backlit. The reddish brown area in the denser tones may be silver sulfiding, where sulfur from residual fixer (sodium thiosulfate) combines with the image silver to form silver sulfide, which has brownish hues. (That's why we wash our film and prints so thoroughly.) I'll be interested in what Mark's conservator wife thinks.
It could also be that the photo of the negative simply had the color saturation slid up too high.
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