Horizontal Frame Clamp, Part# 415163, by Matthews Studio Equipment: can be used in any of eight different positions.
There are two plates that you can see in the photo, each 4" x 5 ½". The plates come apart so you can mount them, and the clamp is welded onto the top plate. The four screws are 3½" long and the thread size is 3/8-16.
In the Box
Matthews Studio Equipment 415163 Horizontal Frame Clamp