This looks great. Did you melt the brass into it?
It's Sintered metal which is basically a real metal but in a powdered form. On small pieces like this you just flood the cavity with Ca and pour in the powder. Once dry you can turn or sand back. On bigger holes you can mix it with Epoxy to form a paste and build it up to fill. I have Brass, Aluminium, and Old Silver but you can get copper as well. I believe Turners retreat stock some.