What kind of horse bit is this?

I an 99% sure you are refering to the Wilson snaffle, a rare bit predominately used in driving, though I have seen it in leading rein ponies and also hunting and eventing.

It can be quite a severe bit with literally, as you say, two rings on each side of the mouth piece. Generally the reins are attached to the 'inside' rings on each side and the reins to the outer rings, therefore the inner rings act as a fulcrum (The point or support on which a lever pivots) which in turn increases the effectiveness of rein aids.

As to the mouthpiece, almost every type of bit comes in the 'mullen mouth' design which refers to the curved mouthpiece. Some english bit designers also take pride in designing more curved mouthpieces claiming it reduces pain in the mouth of the horse. The brass coating, is also avaliable in almost any bit. This attributes to little of the bit type.

I hope this has helped!

A picture of the bit is here:

http://www.thesaddleryshop.co.uk/images/newimages/...

Maybe it's a walking horse bit? Sounds like an english training curb bit used for doubled bitted bridles (saddle seat or dressage) but a mild version for a horse too sensitive to the curb?

Well it sounds like a copper covered twisted W Y bit.

It's either a wilson snaffle used for driving, or a scorrier a severe kind of snaffle for riding. from your description I can't tell which but look them up and you'll see which it is.

do you have a picture? It might just be a regular twisted snaffle with ring stoppers instead of rubber ones.

sounds like a mullen mouth gag

picture would really help.