Put the rear sight assembly in a gallon freezer bags when you pull the broken screw out, by the head, as it's easy to forget about it (plunger & spring) & it'll fly off, likely to be lost.

The elevation screw should not be turned/removed thus it's plunger & spring should be not be a problem.

To remove the rear sight assembly you remove the forward screw only. The rear screw (elevation) does not secure the sight to the frame.

You can use a small jeweler's screwdriver to snug up the new windage nut in lieu of the forked windage nut mentioned.

The nut end of the screw is hollow & once you are ready (see instructions) to flare it (so the nut won't/can't come off when the screw head is turned) you can use a center punch for that.

It'll make better sense once you have the new parts in hand & can look them over. You might want to order an extra windage screw & nut too (don't need another blade) just in case you need to redo it.

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Waiting for the break of day