Sorry guys. I bought it this way, so I can't answer your questions. I'll be happy to take higher res pictures if you need them. The arm sits on a 2.5" by 2.5" by .5" square of aluminum that is screwed to the right rear corner of the plinth above the original oval shaped arm hole. There appears to be one hole through the aluminum square that the arm fits in.
Glad to hear I could sub in a better arm.
Glad to hear I could sub in a better arm.