I just finished restoring my horn and wanted to add my comments. In addition to the above information, you must also cover the three prongs on the "lock ring" ( clip piece in the first picture ) with electric tape or the horn will blow when you put it together and turn on the key.
I replaced the rubber pad, put the horn ring on next, which had the inner plate attached, then the metal cup piece which rests on the 3 prongs on the plate. The nut holds it all together. But no horn no matter how hard I pressed. I verified that there was juice however coming from the button on the end of the wire. (I also added a brass washer on the button to make for a tighter fit for the lock ring.)
I discovered that the horn wouldn't toot because the gap between the bottom of the lock ring and the top of the metal cup was too big to make contact when I pressed the horn ring.
The solution was simple. Narrow the gap by bending UP the 3 prongs inside the horn ring that the metal cup sits on. It's best to detach the plate from the horn ring so you won't break any pot metal. Bend all three SLIGHTLY up and All Evenly or the horn ring will sit crooked.
What result you ask? Now, a slight push on the horn ring will give a pleasing toot. This adjustment is not mentioned anywhere in the shop manual. Let me know if anyone needs clarification.