This is a new one on me. The "Slot" that NE mentioned is machined into the bottom of the rib extension, from the pictures it is not present on this gun. Likewise the top of the extractor is flat, it does not have the projection that fits in the slot. As the top projection riding in the slot prevents the extractor from Rocking none of mine have the guide pin above the extractor stem. I cannot recall having ever seen a Lefever extractor set up this way, but obviously this one was.
As NE said this type has either just a screw in the slot or in most cases a T shaped block set in. In this case a cross cut is milled across the bottom of the rib extension & the narrow end of the T fills the slot to the back of the extension & the wide part fills the cross cut. The extractor is thus stopped by the top projection bumping into either the screw head or the block.
