Differences between the various Magnabass pickups?

Hi all

I was wondering what the differences are between the different Magnabass pickups? I'm guessing that the IV version is geared towards being a bridge pickup, but I wasn't sure what the differences are between versions I, II and III. Is it perhaps based on production year, or is it based on their output? I have a Spectrum DX and I'm wondering whether an ST is 'quieter'.