No idea what the visible difference is, maybe some sort of anodised finish, or a stamp to identify them as coated.

But they must have identified them in some way, and been able to prove this to the team, or there surely would have been an appeal.

I feel sorry for Michael, there was no performance gain to speak of, it was simply an out of spec component, as scaramanga says, they made some changes a couple of years ago to try and keep the playing field level, and this was probably a just a small part in a chain of those changes, which on its own is probably done purely to aid longevity, which you think would actually keep costs down.

It does seem harsh, but look at MotoGP now, we are even seeing penalties given for incorrect tyre pressures, so a tiny bit of wheel rim or valve seepage could cost you dearly!!