Weird set screw on single handle bathroom faucet

The next step secret to getting more of this faucet apart are the two slots cut across the cupped part. See the arrows in the following picture.

It looks to me as that the cupped part is threaded down inside the larger diameter part that sticks up from the hole in the sink. There is probably a special tool used to engage these slots to thread the part out. It may be possible to lay a piece of metal bar stock across these slots as a tool. If you do that make sure that it has nice square corners.

It seems that Moen has an outstanding customer support, and after emailing them a couple pictures of my faucet to confirm the model they emailed me back instructions to replace the cartridge (along with a confirmation of the cartridge my model was using).

S o basically:

1. Turn water supply off
2. Use an allen wrench to remove the faucet's handle
3. Grab the collar and rotate counter-clockwise.
4. Using large pliers or channel locks remove the retainer counter-clock wise
5. Using pliers grab the white square of the cartridge and remove it from the faucet.

Documentation says that your bare hands should be enough for step #3. This was one of my problems I wasn't sure if I was going to need any tools. It seems that rotating should be enough and force can be applied.

To install the new cartridge you need to pay attention to the cartridge's 3 holes and 2 small tabs protruding. You need to look into the brass chamber to find the location of the 3 big and 2 smaller holes, align the cartridge and insert it making sure you are lining up the three big holes.

Gentle pressure should be able to rotate the cartridge.

After making sure that the cartridge is positioned correctly, you reassemble the faucet.