It was a plate, probably 3" "tall" and as wide as the frame, attached in four places at each of the corners. As a result it held the frame rigid (much like a co

It was a plate, probably 3" "tall" and as wide as the frame, attached in four places at each of the corners. As a result it held the frame rigid (much like a corner bracket would). If you pulled the top toward each other, in addition to the top bar of the frame resisting that pull, the bracket would resist and attempt to make the bottom/side angle less than 90 degrees.