Thanks for the replies, everyone. I've contacted Dover but haven't found what I want through them. As for plastic, it's doesn't really suit this horses needs as well as rubber does. Rubber is really our best option for him. I have looked at the happy mouths, but all the ones I've seen have been too thick for my purpose. Sara, I've come across the bit in your link. It's the one that's come closest to my specifications, but it's still a touch thick. And you're not kidding about the 5 1/4, lol.