Nathe bits, as ThreeFigs said, are also legal. I emailed USEF about using the first bit at a show last year and they told me that I needed the smooth bit rather than the one with bumps. Just wanted to throw that out there because I wouldn't want anyone else making that mistake!
Yeah, I didn't know either! I don't quite understand why they are, but I'll just go with it. I'm not exclusively riding dressage (we're working on jumping, too), so I still use mine for schooling and save my Nathe for dressage shows.
Thank you so much for the compliment on Ham! He's definitely a character and lives up to his name. I can't wait to get him out showing this year!