Fiberon 45 degree Support For Longer Bottom Rail Brackets
It appears there are 2 sizes of Fiberon Attachments. A shorter (measured from short end bottom to top) and a more elongated design. This is a remix of the shorter to accommodate the height of the longer piece.
Update: The reason there are height differences is to accommodate different steepness of stair rails. The longer size here can accommodate a much steeper rail. Just found that out. The shorter one is for straight runs and limited angles on the railing.