ATTN: Manfrotto 393 users
I wanted to share some info that might save you some trouble. The thumbscrew that you tighten when securing the QR clamp to the QR plate is brass. Why? I don't know. Anyway, I broke mine recently in the field. Obviously, it was a pain since the 500mm won't carry or support itself. I contacted Manfrotto, they told me to call the parts department and order a new one. I ordered 2 replacement assemblies reasonably priced at $12 something apiece- one to install and the other for a spare.
So if you're using this head, you might want to go ahead and contact them and keep a spare in your bag. It's very soft metal and obviously the weakest link in the chain. Avoid unecessary loss of time in the field!
Hope this helps someone!