This is a replacement Aluminium lock ring for Chris King's R45 Rear Campagnolo Hub
. The R45 will not work with a standard (offered by Campagnolo) lock ring. The sizing of the ring is specific to your smallest cog (11T or 12T). Although the 11T lock ring will fit in a 12T cassette it is highly recommended that you buy the 12T lock ring and don't try and run it this way.