A spade bit is a bit used in the vaquero tradition. It has long, straight shanks, often highly decorated. It also has a straight mouthpiece with a high piece in the center (called a port) and a spoon plate fixed to the port.
Spade bits are used only by skilled riders on highly trained horses (it takes five years of training before a horse is even considered to be remotely ready for a spade bit). It's not the same as a regular curb bit. For any more you'll have to talk to a vaquero, but it's very similar to the full bridle - used only by a skilled rider on a skilled horse.
(For worldbuilding, consider something like this as a fine tradition of equestrianism instead of going with the bridleless trope).