In bitcoin, every coin address is visible, its balance auditable and its continuity with every other address explicit. That transparency is UNIVERSAL and SYMMETRICAL. i.e. it isn't restricted to keyholders, everyone has the same view whether you're a payer, payee, network participant or non-participant.
That's fungibility.
Fungibility doesn't mean that *everyone* can see that coin 1 is different from coin 2. Fungibility means that YOU cannot make the difference between someone offering you coin 1 or coin 2.
For instance, if ever some of Satoshi's bitcoins start to move, they may have "unique collector properties" which render them 20 times more expensive and valuable than your every-day Chinese mined coin. The same way that a handkerchief used by Napoleon may be valued much, much more than your cousin's handkerchief.