Help! Need spare pivot sleeve for Campagnolo rear derailleur

It sounds as if you are possibly fitting it incorrectly - you have to tension the spring, which brings the buffers into the correct orientation.

It's not an easy job without the factory / Service Centre tool, but it is do-able.

1. Put a 5mm allen key in the vice, vertically.
   
2. Fit the bolt into the mech and fit that onto the end of the allen key (so you have both hands free - you will need them)
   
3. Fit the sleeve so it sits on the end of the bolt with the end of the spring in place.
   
4. A small flat-bladed screwdriver can be used, utilising the end of the bolt as a fulcrum, to drive the mounting sleeve around to tension the spring and get the two stops into the correct places.
   
5. Holding that tension on the spring, use the edged side of an engineer's hammer to tap the mounting sleeve down the bolt, until it's snug against the derailleur body.
   
6. Carefully remove the screwdriver whilst holding the mounting sleeve"down". You should be able to see the Seager clip groove in the pivot bolt.
   
7. Fit the Seager clip using a Seager clip tool or at a push, a pair of fine nosed pliers to gently squeeze it into place.
   
   

As I said, not easy. Mind your fingers when doing it!
@Newman8 -
Campag don't usually give the same part different part numbers - there is usually a difference of some sort, although it may not be obvious. If it's a new version of a part, they will change a code like this -
XX-XX00x will change to XX-XX1ox, will change to XX-XX20x and so on.
A good example is Record Ergopower levers - we started at EC-RE047 for the RH. The 12s spare is EC-RE847 ... there have been 8 versions so far. Sometimes it's just been a cosmetic change, sometimes structural.
Sometimes a sub-assembly may be split out into individual parts, or the reverse might happen and in that case, the individual parts (or the sub-assembly) will get a new part number - so when ErgoPower levers went from a metal index spring retaining ring to composite, we went from ordering / fitting EC-RE111, EC-RE057 & 2x EC-RE209 to just ordering the sub-assembly EC-RE065, even though the index springs were the same and he new sub-assembly fitted into the same place previously occupied by 111, 057, etc ...