all of the 733's has the hydro boost brakes, the "booster" is actually a nitro chamber like mercedes has for the self leveling suspension systems.
it attaches to the bottom connection block in front of the master cyl, i think all bmw's of that era had it except for 3 series 533,633,733,535, 635 and 735 US and worldwide models. i will see if i can get you a picture this evening from mine

below link shows the accumulator and connection block where the accumulator attaches

http://images.google.com/imgres?imgurl=http://www.************/assets/product_images/hardpart/34331152825.jpg&imgrefurl=http://www.************/se1.asp%3Fdept_id%3D170&h=107&w=200&sz=3&hl=en&start=2&tbnid=omVXTuqy9C3otM:&tbnh=56&tbnw=104&prev=/images%3Fq%3D733i%2Bbrake%2Bbooster%26svnum%3D10%26hl%3Den%26lr%3D%26safe%3Doff%26rls%3Dcom.microsof t:en-US

http://images.google.com/imgres?imgurl=http://www.************/assets/product_images/hardpart/34331152825.jpg&imgrefurl=http://www.************/se1.asp%3Fdept_id%3D170&h=107&w=200&sz=3&hl=en&start=2&tbnid=omVXTuqy9C3otM:&tbnh=56&tbnw=104&prev=/images%3Fq%3D733i%2Bbrake%2Bbooster%26svnum%3D10%26hl%3Den%26lr%3D%26safe%3Doff%26rls%3Dcom.microsof t:en-US