This is a genuine BMW securing pin, part number 52 23 7 004 728. The number stamped on the part is 52237004728; BMW's catalog files it under the short number 7004728. You will find it on our Interior Body shelf, under Other Seats parts.
It fits the E46 sedan and touring (320i, 323i, 325i, 328i and 330i, 1999–2005), with the M52 or M54 engines — and the European-market versions.
BMW draws it on 2 diagrams, among them Seat, rear, cushion, & cover, basic seat (52_2615) and Seat, rear, cushion, & cover, basic seat (52_2788). BMW's catalog calls for two of these at each place it is fitted. It weighs about 6 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.
