The Tektronix Type P plug-in is a calibration aid introduced in 1959 for 500-series scopes that applies a fast (4 nanosecond risetime) voltage step to the mainframe vertical amplifier via plug-in connector pins 1 and 3.
Type P is designed to simulate a perfectly-adjusted Type K plug-in being driven by a Type 107 square-wave generator, and is used when adjusting the peaking coils and the delay line. The idea is to calibrate a mainframe using Type P, then use this mainframe and a 107 to calibrate other plug-ins.
The Type P does not contain any tubes, transistors, or tunnel diodes. It does contain a selenium rectifier and a mercury switch. The selenium rectifier in the Type P has four sections and full-wave rectifies the 6.3 VAC that is supplied by the scope mainframe on pins 13 and 14. The 120 Hz (assuming 60 Hz mains) component of the full-wave rectified waveform is applied to the coil of the mercury switch to drive the opening and closing of the contacts. An adjustable fraction of the DC component of the full-wave rectified waveform is applied to the coil of the mercury switch to create a constant component in the magnetic field to bias the switch contacts to the best operating point.