could be
Dim3nsioneer 557
If the knurled nut is zinc-plated and the screw is made out of stainless steel then the knurled nut will soon show rust when the two are kept in an environment with high humidity. It's a question of the standard electrode potential.
Copper grease is applied to those parts during assembly; I think that may be what you're seeing.
Mine looks the same.
Ok so i can be pacific.
copper paste!!! mine too... thats nothing to worry about !
Ian 
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IRobertI 520
I haven't looked at it closely but I noticed the same thing myself. However, I think it might actually be something like copper paste for lubrication rather than rust.
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