This is for a heavy duty cast iron and steel ladder rack system. I had it on my Chevy one ton and than a Ford E-250 van. I cannot tell you if they are rated for 300, 400, or 600 lbs. I can tell you that they worked for me for ten years with multiple extension ladders and other items at the same time.
New this type ranges from $120 for something that looks scary / flimsy, to $150 for something closer. Example here;