In article
,
RichA wrote:

What else could it be? Sloppy cleaning on assembly? Could be, static
charge can make plastic crap stick like glue, making it hard to clean-
off, but eventually, that would stop, as would the amount of debris in
the body would diminish with time. That isn't happening, apparently.
Anyone with a spectrometer could collect up some of the stuff and
analyze it but unless Nikon forgot how to apply in-body blackening
paint correctly, (after 80 years) this crap IS plastic.

it could be any number of things. you have no idea whether it's plastic
or not.

sigma had a problem with their shutter mechanisms shedding metal
particles in their early slrs, and those were metal.