The best info we have on Fed's racket is either from Greg Raven or Art Art.
Check Greg's site which shows it to be very similar to the stock wilson tour 90 series.
Art Art's specs from the French Open are also similar to the stock specs of the K90 and he said it appears to be a stock K90 when compared with stock one's on site. Art Art's specs were partially backed up by Ron Yu, who along with Nate Ferguson actually are Fed's stringing and customization company of choice. Though Ron didn't confirm that Art Art was correct he said that his specs were 'close'.
So it appears that either of these are the best evidence that we have on what Fed is using.
IMHO, both Greg and ArtArt are right. Greg has older nCode rackets, and I assume that since then Fed has altered his setup.
I think Fed is pretty much open to tweaking his setup like agassi, how else would he have tinkered his way from his average 24-28kg stringing preference circa 2003 to all the way down to 21 kilos nowadays. I assume that since then he might have altered his weight, and the result is the K90 that we now have? It's up to you but this is the best info I think any of us have towards what Fed is really using.
Be awed at the sight of my prowess.