For a short while, in 1977, a full Campagnolo Grouppo came with a quite rare accessory, to make it street-legal in the USA :
http://www.ece.ubc.ca/
This part was only part of a full grouppo purchase, straight from
Campagnolo, in a boxed grouppo-set. If you bought parts from a
typical american supplier (such as Bike Warehouse or Turin Cyclery
Co-Op), they got their stuff through bike makers oversupply and this
part was not included. These are only available on full grouppos
purchased through the Ochsner USA distributor in 1977 !!!
http://www.ochsnerusa-store.com/
I'd say that this has to be the rarest campagnolo part, hands down! It's the only one I've ever seen !!
- Don :-) Gillies
San Diego, CA, USA