The Type 1A1 plug-in has two channels and a -3dB point of 50MHz at 50mV/div, 28MHz at 5mV/div, and 2Hz to 15MHz at 500uV/div when channels 1 and 2 are cascaded. Early production used a Nuvistor front-end, and rotary input switches that were concentric with the BNC jacks, while later units have FETs and lever switches. Type 1A1 uses the "ALT SWEEP SLAVE PULSE" signal that is sent by scopes on pin 7 of the plug-in connector. In a Type 547 mainframe, this signal allows channel 1 to be displayed using the "A" timebase and channel 2 using the "B" timebase. This is useful, for example, to view the IF and AF of a radio receiver at the same time.
Types 1A1 and 1A2 were introduced together in 1964, the former for high-end use, the latter as an upgrade/replacement for Type CA. Both remained available until the classic plug-in scope line was discontinued after 1974.