The Happy Tongue is a solid mouth piece. It is curved and ported to give tongue relief. The Happy Tongue would be a good place to start if your horse is:
Shaking its head
Sticking its tongue out
Sucking its tongue back and as a result making an intermittent coughing noise
Trying to put its tongue over the bit
Going behind the bit to avoid the contact
Snatching the reins forward and down
The Eggbutt cheekpiece prevents pinching of the lips and give a slightly more solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower and also introduces some poll pressure.
125mm = 5"
135mm = 5 1/4"