I'm not sure whether Edison changed the horn arrangement on the 30 over time.
However, I do have an early one, and can tell you that the horn should have a small rod on the bottom, threaded at the end, that goes in the slot shown. A couple of felt or leather washers should be placed on either side, and a small hex nut to secure it.
Of course, you'll also need a horn suspension spring of the proper size: check with APSCO or Wyatt's for that (specifying a 30)......... Another essential detail is to put a small piece of rubber tubing on the horn's suspension hook, and to slip a (larger diameter) piece over the spring itself. This insulates the horn from the rest of the machine and improves the sound quite noticeably!
Hope this is helpful.
Bill
