This is a Manfotto (Bogen) super bracket, it's one of the many super brackets they make to support and attach lights or softboxes or umbrellas, you can even see by looking at the photo that it can do all three jobs. It has a pivot in the centre, a slot which has a clamp for holding an umbrella at an angle, and the brass threaded adapter.

I'm sure you know the deal with these super brackets, basically this goes on top of your light stand, and everything then attaches to it. Very simple, neat and strong. Made in Italy. Never even once used this one in my studio, so I may as well get rid of it. They are $40 or $50 in the stores, and it is still new.