Originally Posted by Andy_K

For use with 8-speed Campy shifters, you'll want either an 8-speed Campy rear derailleur or an "old" 9-speed. The 9-speed thing can get a bit confusing because they changed the cable pull in the middle of the 9-speed era. I believe any Campy 9-speed rear derailleur made before 2001 will work for you, while any 10-speed rear derailleur won't. The way that you can distinguish between the old cable pull and the new cable pull is by looking for where the B-screw is. If the B-screw is next to the derailleur mounting bolt (similar to where Shimano puts it) it's going to use the old cable pull. If the B-screw is next to the upper jockey wheel, it's going to use the new cable pull. Looking for that will save you having to figure out what was 8-speed and what was 9-speed, I think.