upper control arm

hi every one

I was looking at ebay and found those
http://cgi.cafr.ebay.ca/73-76-Plymo...r_Truck_Parts_Accessories&hash=item2eb632030d

one thing I dont understand is why it shouldnt fit
front disc brake I alwais think that big ball joint was
for 1973 and newer A body with or with out disc brake

can someone explain this to me

thanks Bob