|W. Ehrhardt Full Plate Keyless. 1920|
This design makes the watch rather more robust, slightly slimmer (although it is still considerably thicker than a three quarter plate) and makes it very easy to get the watch "in beat".
Being in beat means that the balance action is symmetrical swinging an equal distance in each direction and that the balance staff, impulse jewel and pallet arbour are in perfect alignment. If you think in terms of a long case clock the "tick" equals the "tock" and it has a steady rhythm.
This can normally be a tricky operation involving quite a lot of work and often much fiddling with the hairspring which always has some risk attached to it. With this design the impulse jewel can be lined up simply by rotating the balance cock to the correct position and locked there with the two screws that hold it to the movement.
Unfortunately it did little for Ehrhardt who was out of business shortly after, or for watch design as the full plate movement was already obsolete by the time this was made, replaced by the three-quarter and split plate movements.