Originally Posted by Aldone

Wait, wait.

If the cog is this one (photo taken from e-bay), then the cog IS NOT for fixed gear and it doesn't come from a Pista Campy Hub (I know bacause I have several old campy track cog, they're all stamped "Campagnolo" with markings of teeth number and threat and they're very different)

I think this cog is from an OLD screw-on cassette (sorry I don't know the exact name in english), the "hole" is much larger than on a fixed gear cog and doesn't fit on a track hub