I think you are going to be stuck then as the Nathe is the closest I think you are going to get. What exactly is wrong with it?
If the Pelham version of the Nathe is like the loose ring version, it's flexible. I'm not prepared to spend that much money for a flexible bit without knowing how the horse goes in a bit that flexible.
As far as diameter - exactly what SmartAlex said.
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Not sure if this would help, but I've seen people use a regular metal pelham, then wrap the metal with latex wrap. The latex wrap comes in a small green box and is available at tack shops.
That would be Sealtex brand latex It would be my choice as you can make it as narrow as you'd like. If you're worried about thickness, you can wrap the bit more creatively so that the sides are thicker than the middle, allowing more room for Pony's tongue.
Have you tried a Myler fixed mouth with the hooks to give you a little leverage? It's jointed but only swivels so nothing bumps the the roof of the mouth.
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