If you are talking about the over-axle y-piece, then this is it and its number is WCE000711. When they were available from Rimmer's, the Rimmer's diagram cross referenced the mid-section and the over axle section causing some confusion!
WCE000711.jpg
... but I think they have bee NLA for over 3 years. There was once a discussion here about having some made up but i think it came to naught, see: https://www.two-sixties.co.uk/mgoc260/html/f0rum/showthread.php?17249-Possible-exhaust-supplier
David