 They were characterized by similar appearance, tuning drills, frequency indication using film strip displays, the use of relatively simple architecture that avoided complex switching as far as possible and using commonly available components and a degree of modularity in construction.Plessey Mk.IV connectors were used extensively in the sets and accessories.An important operational advance was that the sets incorporated internal calibration facilities which meant that they could be accurately pre-set on a frequency without radiating any signal.