The ruler part needs to sandwich the bracket between itself and the slide for the paper fingers at the back. The bracket helpfully rotates in a wilful way, while you try to get the sandwich together and keep the spring from falling off again. I considered the possibility of taking photographs of my mini disaster midway through the screwing of the screw and thought, well there are limits to my dexterity! The accidental magnetisation of the screwdriver on a previous occasion was a boon to this little fix.
So back to the typing. Did I succeed in eradicating the dirty smears? Unfortunately not so much. Back to making eraser crumbs, then!