They come in 135 mm mouth size only.
Trust Leather Covered Eggbutt
This eggbutt has a 5/8″ flexible, leather covered flexible mouthpiece.
Leather mouthpieces are a bit of the magic “unicorn” of mouthpieces. When all other mouthpieces have failed, this one could be what you are looking for, especially if your horse doesn’t go well in plastic or metal bits.
Made in Germany