The horn on a 1975. Can those nuts be removed, so the two terminals can be cleaned underneath?

Yes but that is not usually the problem. There is a set of contact points inside that get corrored. You have to drill out the rivets and take the cover off to a cess them.

The earlier set I have, has rivets, but my later one does have any rivets. Almost looks like it is crimped on around the edges, similar to the gauges? As of a year ago, it worked, and so as a last resort, I was going to see if that would help.