IF, and I repeat IF, they are "illegal", I'm sure its like Brad said. At some point they were probably not legal to install by a manufacturer. That said, back in about 2004, I was looking for parts for one during a trailer resto project. I spoke with Competitive and a few other trailer builders and not one said anything about that style coupler being illegal. Just stated they had not been produced for years and parts weren't available. Honestly, I bet they are a lot more expensive to manufacture than the typical "drop-on" style. They use two forgings, a welded tube and then an arm welded on. Compare that to the typical one-piece stamping of the newer style that has only one forged piece(if that).
2sav, measure that trailer tube before you go jumping into a coupler swap. I bet it is 3.5" square. There aren't any modern couplers that are designed for that. 3", 2.5", 2" are all I could ever find.