That test if for an internally regulated alternator. On a 240Z w/external regulator the test is basically the same except the positive probe goes on the 'A' terminal. The difference is that the test won't tell you which is bad, alt or reg.

I would bet the extra wire (in post #1) coming off the "A" post is/was the lead to a noise Suppression capacitor that has been broken. See the picture in post #6. I know the wire is blue in that picture but, on my 70 it is black. If you do not, have noise problems with your radio just remove ithe wire. If you have noise problems replace the capacitor. Take the alternator off and have it load tested. Then you will know if the alternator is bad or if the problem is in the wiring or voltage regulator.

The broken wire in post #1 is coming off the "E" post as shown in post #5 so a capacitor should not have been connected there. But it does appear in post #1 that the ground wire is connected though the rubber cover is not over the terminal. But I agree-test the alternator.