51 21 004 Adjusting front left or right door detent (lock wedge)
WARNING: This page is about a different car, the 2010 BMW M3, 2009 BMW M3, and 2008 BMW M3. However, it is still accessible from the selected car via links, so may be relevant.
Operation is described in:
Adjusting left or right front door. See 41 5. ... ADJUSTING DOOR .