I got one also,I did ask about it a while back on another site.Most thought that it had been grafted by someone,ya right
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This is all I know about this,or should say was told-
The 70 style 4 screw top plate would actually vibrate and make noise when you had the windows down and the 5 screw set up was to cure that issue.Wether that is the truth or not?Heres a couple of pics to see for yourself.