Campagnolo changed the RD geometry in 2000, and the difference is clearly visible. The RD's made until 1999 had a fixed stop on top and a stop cam with the typical B-screw adjustment. Starting in 2000, with the introduction of 10s, the entire line changed to a double spring system without a B-screw. The mounting angle was adjusted by changing the relative tension of the lower and upper springs, via a worm gear screw on the lower body.

I tried finding photos showing the difference but couldn't quickly find any that showed the are clearly enough. You'll certainly know if you have it in your hands, but might ask the seller about the B-screw and the answer should make it clear which he's selling.

Edit-- I later found this photo of a later carbon Chorus RD clearly showing the angle adjustment on the bottom.