This part is listed by BMW as BMW part number 33321127717 (33 32 1 127 717) and is described as Genuine BMW Swing Support Bracket, better described as the rear sway bar mounting bracket, found on all E30 3 series, E28 5 series (1982-1988), E36 318ti (1995-1999), and first generation BMW Z3 (1996-2002).
