Dilly541,
The "rim screw" you mentioned is what I called the "headspacing screw". The rim recess is radiused, usually, at the bottom. The screw is usually "square" at the bottom, so the object of the "spotfacer" I mentioned is make a "seat" to fit the screw. A seat can be made with other tools, such as scraper, graver, burr in a Dremel, etc. The main thing is to remove the radius, where the screw is( removing it completely would increase headspace). I have seen then fitted by filing the bottom of the screw where it contacts the rim recess. It worked, it's just not "elegant". As far as the notch in the extractor is concerned, you are correct that it is fairly straight forward. I suggest filing or sawing it too narrow and then "file and fit" a little at a time with a safe edge swiss file, to a close fit, before fitting the headspace/rim screw. This makes locating the area to be worked easier. Notice the "witness mark" on the EL. The clear intention is to make a corresponding one on the main extractor. Unless you drive the roll pin out, I find this unnecessary. Since the EL seems to be used, there is a fair chance that its "regulation" is not too far off. I suggest boresighting it after fitting, to see how far off it is. When(if) adjusting the POI, if it seems to impact backward from the adjustment, the adjustment may have been too much and the bullet is hitting the barrel. Don't ask how I know.
Mike
