Afternoon Ruston
Its part of the mount for the coupling & its purpose is to ensure the hook wire returns to the correct height, centrally & allows sideways deflection when engaging with another coupling.
See this post for more detail,
http://www.rmweb.co.uk/community/index.php?/topic/90024-the-mill/page-16&do=findComment&comment=1882039
Cheers
Chris