campy & brooks collaborated on a special narrow-railed saddle and seatpost to match..
(along the lines of the narrow-railed keirin seatposts and saddles, if you're familiar with them)

the part for sale is not the important one, it's the upper cradle parts and post that matter when fitting the narrow rails
(it seems like the seller only has one piece)

also, the special saddles were stamped campagnolo where you put your butt

all this stuff is very rare, but not much good unless you've got all parts of the equation