The safety chains on my horse trailer are mounted onto the trailer through a strange U shaped, unthreaded, piece of steel. I want to replace my safety chains and looking for a mounting system to the trailer.
The new mounting hardware should replace these unique connections. The hole is about 3/8" in the trailer where these clamps go.
Take the old chains off with cutting torch like this... Or maybe use a "sawzall", disc grinder/cutting wheel or a very very very large set of bolt cutters.
You may want to take it to your local welding shop and have them R&R in the correct manner.