Lowlevlflyer,
Those are actually a building aid that I made for when I was still using pins to build with. I took a piece of 1" extruded aluminum that I picked up at Lowe's. I cut them into 1" pieces for this use. For using pins I drilled a hole on one side of the bracket to pin it down with. When I converted to magnets I put some adhesive magnetic strips on the bottom of the bracket but it doesn't have enough "grab" to stay in place. So I use 2 magnets on each bracket to hold them down for this build. Once important thing is that after you cut the 1" pieces it's a good idea to check each bracket with a square to ensure that it's still at 90°. The biggest place I use these is for holding ribs in place while building the wing. Take a look at these pictures and let me know if you have any questions.

Ken