Eggbutt Lock Up from Bombers.

Eggbutt prevents the horse from being pinched in the lips/corners of the mouth by the bit rings and provides a more stable feeling against the horse. Eggbutt ensures that the bit remains still in the horse's mouth.
The Lock Up mouthpiece prevents the nutcracker effect of regular jointed bits, reducing pressure on the bars. Lock Up is therefore milder than a regular jointed bit, but it still applies pressure on the tongue and bars when the reins are shortened.