This is my favorite lack stacking bracket design. The screw holes fit #6 wood screws perfectly.
A couple of them were really hard to slide onto the legs - had to file the inside very lightly in a couple spots. The rest fit perfectly, so I don't think it's the part's problem, just the legs not being perfectly square.
When screwing them down, I recommend fitting all the legs, then holding the bracket tight against the edge of the bottom table while screwing in the bottom screws. That helps make sure the legs of the top table are perfectly straight while you lock it down.