This will probably only add to the mystery, but I bought a low mileage 990 that had weird deposits in the intake tracts.
I never did see the throttle bodies of the bike it came out of, but the intakes looked like this:
I brushed it out as well as I could, then ran it several thousand miles with carburetors. When I pulled it apart for my first valve adjustment, the intakes were clean as could be.
I have seen similar pics posted, and often the condition is blamed on ethanol fuel. But, almost all of the fuel I use is E10. so I doubt that theory.