Just like on most other pups with adjustable poles, you can raise or lower them to affect the volume of a given string. I am not going to pretend I understand the physics behind it or why that works, but it does. I would guess that it extends the magnetic field so that the string is in a stronger portion of it vs weaker periphery. ... I guess it's also like raising the entire bridge pickup a bit or dropping the neck to balance out their relative output.
The screws on mine are magnetic (even though they are mounted into a place, as you say, between the coils and the magnets on the outer edges of those coils - there could be a 3rd magnet or just by virtue of metallic contact between the center rail and the outer magnets - there is a baseplate - though if the magnets are strong enough that would not be necessary). I have a reissue (out of a bud's Epi EB0) in the parts drawer that I could check as well. ... oh wait, if you look at the pics here (
http://music-electronics-forum.com/t28235-3/) you can see the core sheet steel that runs through both coils and touches the outer magnets. There's also that thing on the bottom middle that could be a rusty ferrous magnet, but I am not sure.