This is a genuine BMW square nut, part number 07 11 9 921 230. The number stamped on the part is 07119921230; BMW's catalog files it under the short number 9921230. You will find it on our Supplies & Hardware shelf, under Group 11. BMW has since replaced it with 9904050.
It fits the E46 convertible (323i, 325i, 330i and M3, 2000–2006), with the M52, M54 or S54 engines — and the European-market versions.
BMW draws it on the Front Panel (51_4115) diagram. BMW's catalog calls for six of these at each place it is fitted. It weighs about 3 g (under 0.1 lb).
We have not had one on the shelf yet. Request it and we will tell you when one comes off a car.