The actuator back-block holds the actuator rods in their off-normal positions during phases 2, 3, and 4, of the operating cycle.
The actuator back block is composed of one gray strut per control frame. Between control frames, the structs are connected by orange (arity 2) connectors, which are mounted to the control frame by pairs of gray (arity 1) connectors coupled by green struts.
In the picture below, the actuator back block is shown mounted on the control frame.
The right hand end of the actuator back block attatches to the back block lifter.
At its left hand end, and at any intermediate points between control stages, the back block is mounted via the two gray (arity 1) connectors coupled by a green strut.