Only (some) DC horns had an adjusting screw. AC horns don't need one. The horn makes it's noise by vibrating a steel diaphragm. Here is how a DC horn works:
https://www.autocurious.com/2019/02/how ... -work.htmlThere is also a spring in there working on the contact points. On some horns you could adjust the tone by changing the spring tension.
An AC horn has no points. It moves the plunger back and forth just by the fact that the coil's polarity changes every time the alternating current flips. If an AC horn is hooked to DC current, it emits a single click as the plunger moves all the way one way and then stays there.
Back to where we were. It's all working fine and you have an AC horn and are running on the regulated (but not rectified) AC output.