Bubba13 is correct, it is a Kimberwick. Without the curb chain it will function like a boucher with very slight pressure on the mouth and a tiny bit of pressure on the poll under the headstall. Most horses will not react to the pressure and will feel like a mullen mouth D ring with a low port. A sensitive horse will "feel" the difference. If you were to add the second curb hook and curb chain you would have a true curb bit with slight poll and jaw pressure. It obviously has some age to it and I wouldn't be surprised if it were made of nickle and not stainless.
A snaffle bit is a bit which has no curb (chain or strap which exerts pressure on the jaw and poll) action no matter the mouth piece. Snaffle bits can either have a straight mouth piece (mullen mouth/when covered in rubber known as a dog bone), a ported mouth piece (mullen with a raised section in the middle), a single joint or a double joint with a center piece in various shapes. In the US we have all gotten lazy and refer to any jointed bit as a snaffle. I am trying to remind people of it because I watched a lovely young rider DQ'ed at a dressage show when her "trainer" sent her in a snaffle bit which was actually a jointed Tom Thumb which is in reality a curb bit. I always refer back to the Pony Club manual (both the US version and the British Horse Society version) on my bit identification. Not intending any disrespect anywhere just offering some helpful information.