I believe its a very rusty (or made of something I don't know of) half-cheek mullen mouth snaffle. It is used for driving a lot for sure, but you can also ride in it. I use one on my miniature horse mare, but it's made of stainless steel with german silver inlays.
Similar half spoon-cheek bits were used on trotters and pacers a lot in the UK. I've never seen a mouthpiece like this but I suspect the discs either side of the 'port' were intended to keep it central over the tongue.