Which type of actuator is NOT commonly referenced?

The correct choice is Bristol. This is because "Bristol" is not a widely recognized type of actuator in plumbing or control systems. Common actuators are often associated with well-known brands or types that serve specific functions in various systems. In contrast, Brinwell and Rockwell are names related to types of actuators or systems that are acknowledged in industry terminology.

Brinwell and Rockwell actuators are types of products that have been utilized in various applications, making them relevant in discussions about actuators in plumbing and mechanical systems. Therefore, the prevalence of the Brinwell and Rockwell names contributes to their recognition, whereas Bristol does not have the same level of industry familiarity, rendering it the less common reference among the options provided.