Did you check to see if you have continuity going from the connector on the exterior of the horn through to the inside? A common place to lose it is here, under this rivet head.

That is not a washer, it's corrosion.

@Patcon have you had them apart at any point? If so, is it possible you mixed parts? The left and the right have slightly different internal parts. The bolt thing at the left side in this picture is different for the high vs low horn. You may need to have someone hold the horn button down while you slowly turn the screw until it makes some noise. Also, the horns won't work unless you ground them at the mounting tab.

Thanks for every bodies feedback.

So I tested the body with a volt meter and the is continuity from the power in blade to the case.

I was also very careful to keep the horn parts separate.

I marked one body with 1 and one body with a 2 and kept the corresponding parts in separate numbered ziploks while the bodies went to plating

I'm trying to fix the horns on my 240Z restored by @motorman7 in 2019. I recall that motorman7 found a pair of NIB horns and they certainly look like it. But they quit working some time ago and I'm trying to analyze the problem. Took one apart and it appears to be all OK (photos attached).

When testing horn I have opened up, the diaphragm pops in and stays there. Never vibrates. The other horn acts the same way. I did experiment with the tone screw on the assembled and at one extreme the horn diaphragm pops once, and at the other extreme, the horn does nothing.

Fiber washer appears correct.

So I noticed some corrosion around the wire from the 12v lead in to the coil and wondered if there was a short which would mean when voltage was applied, the coil would always be energized, even if the contact points were open.

BUT, since the horn diaphragm does not depress when volts are applied and the tone screw is all the way in (contact points open), it appears there is no short keeping the coil energized. (I wedged some cardboard between the points, just to be sure and tested again. Coil did not energize. Diaphragm did not depress)

I've tried measuring the resistance but have a rinky dink multimeter which seems to show the right numbers, but cannot get consistent readings. (have ordered a better one from Amazon).

Any thoughts?

Regards from the left coast...

The first thing I have to ask: Did you verify you have voltage at the horn when you press the horn button? I had to deal with a fair amount of corrosion on the horn tab in my 73. Also, do you hear the horn relay click?

Also, I found SteveJ's recommendation of 20 ohms to be pretty close. That screw is also very sensitive!! Goes from 3ohms to Kohms very quickly. I found it need adjustments of just a few degrees.

BTW I did get the my one horn working. Now for the other...