This may be where I have trouble. It is the only part of the fuel injection system that is easy to remove.
This regulator has no shims at all, but it the ebay regulator: the one in my car has a single 1 mm shim. And it runs at 2.5 bar system pressure. I wish I could see deeper into the drillings but my working idea is that the small O-ring and larger axial spring are the parts that allow fuel to return to the tank at a given pressure. Thus regulating system pressure.
My other idea aboutb how this works is that the larger O-ring acts to preserve control pressure when the motor turns off. The WUR doesn't send its fuel directly back to the tank. Instead, it comes back to the pressure regulator portion of the FD. This would be the push valve portion.