This ENVE Hub Endcap is for 12mm thru-axle rear Enve hubs. The 21mm length is used when you convert a Shimano HG11 freehub body to a SRAM XDR or Campagnolo N3W model. You replace the driveside thru-axle end cap that came with the Shimano body to replicate the proper hub spacing. It is made from machined aluminum and anodized black, with an etched size indicator (photo may not match the exact option listed here.