I recently bought a Sonnet Cascade Gold, date code A.III, which would place it in the first quarter of 2002. But it's a fake. No harm done because the vendor has agreed to refund the price, but I thought it would be worth describing the diagnostic points and showing some photos. I have seen a couple of poor quality fakes but this one is much better quality.
The first thing is that the cap does not fit very well. It feels as if it is rough and grating - I have a genuine Cascade Silver, and the cap on this slides smoothly on and closes with a satisfying click. Completely different.
The photos show some points. Superficially they look pretty similar. But the lettering on the cap band is cruder, it is offset and not central on the band, the 'III' in the date code are smaller than the 'A', the symbol before PARKER is the curly P and not the oval with an arrow - and the serif on the S in Sonnet is horizontal not vertical.
The feathers on the top of the clip are different. On the real pen they are incised, deeper and wider in the centre and tapering to nothing at each end. On the fake they are cruder, and the 'gold' has been applied after the feather were stamped, so they look more blobby and rounded.
Finally, on the other side of the band the P and W stamps are larger and cruder in the fake. And the waves in the cascade pattern are subtly different. In the fake the tops are pointy, but in the real one they are more rounded and assymetrical. This varies a bit depending on angle of view but there are definite differences.
I hope this helps others not to make the mistake I made. Good luck!