Never seen these before, are they retro-fit?
http://www.autotrader.co.uk/classified/advert/201302285547611/sort/pricedesc/usedcars/engine-size-cars/3l_to_3-9l/transmission/manual/body-type/convertible/model/z4/make/bmw/onesearchad/used/onesearchad/nearlynew/onesearchad/new/page/3/postcode/l334jb/radius/1500?logcode=p
http://www.autotrader.co.uk/classified/advert/201302285547611/sort/pricedesc/usedcars/engine-size-cars/3l_to_3-9l/transmission/manual/body-type/convertible/model/z4/make/bmw/onesearchad/used/onesearchad/nearlynew/onesearchad/new/page/3/postcode/l334jb/radius/1500?logcode=p