Stainless Steel with loose rings. The wings not only prevent the bit from being pulled through the horse's mouth, but exert lip pressure when the reins are used.
This is a very popular all-round bit. It should not be confused with the USA version of the Tom Thumb, which is a completely different model bit.
This is the best sandle ever made and I loved them so much I bought a spare pair as I can't buy them any more. So disappointed that they're discontinued, a fantastic, comfortable shoe.