Well, first of all, we know these cars better than does Rock Auto. I always look up parts numbers elsewhere as a double check on what they show.
Second, you should learn to use RealOEM. You need to look up your car by production date - not by VIN. Its a long story, but the short version is when auto manufacturers changed to 17 digit VINs, they failed to ensure that previously issued VINs were not duplicated. Thus, by way of example, if I put my VIN into RealOEM - or if I give my VIN to a BMW dealer, which only has 17 digit VIN information in their database - it will point to a 1983 Euro 323i.
So, obtain the production date for your car, go to RealOEM under classic cars, select 2.5CS - 3.0 CSL, select 3.0 CS, then Euro, and then enter your production month, and then select manual for the transmission. If you car was manufactured in April, 1972, you would then find this for the clutch slave.
Note that the slave that matches the Rock Auto part does not show a part number. That is because this slave was not installed in your car.