Not sure if this is the part I am thinking of, but for those who have fitted the ///M bumper on a non-///M I believe you need another part to bridge the gap between the undertray and the bumper - is this it ? If so, someone may want it, if not then just ignore me 
http://www.ebay.co.uk/itm/BMW-Z4-M-...CarParts_SM&hash=item3a75289161#ht_2844wt_689
http://www.ebay.co.uk/itm/BMW-Z4-M-...CarParts_SM&hash=item3a75289161#ht_2844wt_689