A Style M3A machine, which is illustrated above consists of three compartments. The motor compartment contains a reversible dc electric motor which drives a crown wheel in the gear box through a clutch and worm drive. The crown wheel drives a crank shaft from the lower end of which is a double sided crank. A roller on the underneath side of the crank engages in a cam slot to move the operating bar from the 'Normal' to the 'Reverse' position when the crank rotates through an angle of 290 degrees (and vice versa). A dumb-hell shaped cam on the upper side of the crank engages between two rollers when fully driven in either direction and drives the slide bar.

The slide bar directly drives the lock bar in the circuit controller compartment. The lock blade which is connected to the switches via a lock rod and stretcher bar has in it two slots. The lock box contains two dogs, the upper one of which can only be engaged in the slot in the lock blade when the latter has been moved to the fully reversed position by the points themselves. A lower lock dog is engaged when the points are in the fully 'Normal' position.

Each switch blade has also attached to it a detector rod of light construction which drives a detector bar. The two bars have cam sections in them in which detector rollers engage, the rollers being mounted on levers to ensure that 'Normal' and 'Reverse' detection cannot be made at the same time. The electrical control and detection contacts are cantilevered from two terminal blocks lying one on each side of a cam shaft as shown above. The cam shaft is driven by a nylon gear wheel at its centre via an idler gear beneath it by a rack mounted on the rear of the lock box. There are eight pairs of contacts. The inner pairs are used to cut off the supply to the motor as it nears the end of its stroke and those outside it are 'snubbing' contacts to slow the machine down at the end of its stroke. All of these contacts are made when a conducting strip on the cam shaft connects the two halves of the contact. The outer contacts are used for detection purposes and are designed differently. A pair of contact springs are provided, the detection circuit being made when the upper moving contact springs are resting on the lower fixed contacts. When the machine is in its midstroke position the moving springs of the detection contacts are held up by a roller fixed under each pair of springs. This roller rests on a recessed cam on the cam shaft. The end of a push rod, which t transmits the motion of the detector roller, rests in a recess of the cam in such a way that both rotation of the cam and withdrawal of the pushrod are necessary before the roller can drop. When one or other of the detector rollers enters the notches in the detector blades, thus proving the points to be correctly closed, this movement withdraws the pushrod. As the locking movement proceeds, rotation of the camshaft brings a cut-away portion of the cam beneath the roller. Completion of both of these movements allows the roller to drop and the detection contacts to close, thus proving, simultaneously, both the position of the point blades and the position of the facing point lock.