I am assuming it is like other horns. The screw does adjust the horn - but not the electrics, but the gap between the metal plate and the energize/de-energize magnet that causes the plate to vibrate - thus creating the sound and the pitch. By adjusting the gap, you can get the horn pitch (and some loudness) to change. However, it can be touchy, as adjusting too far either way (too close a gap, or too big a gap) will cause the horn not to work.
"The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers