It's called a bridge movement. First saw these a few years ago. From wiki: "Bridge movement In modern watch movements, the back plate is replaced with a series of plates or bars, called bridges. This makes servicing the movement easier, since individual bridges and the wheels they support can be removed and installed without disturbing the rest of the movement. The first bridge movements, in Swiss pocketwatches from around 1900, had three parallel bar bridges to support the three wheels of the going train. This style is called a three finger or Geneva movement."