The Tektronix Type 3B4 is a timebase plug-in for 560-series scopes. It was introduced in 1964.
It was intended primarily for use with the 561A, but works fine in other 560-series scopes. The 3B4 is unusual in having an explicit "FREE RUN" mode.
In the triggered modes, the tunnel diode-based triggering is rated to work to 20 MHz. The vertical bandwidth of typical 560-series (non-sampling) scopes is about 10 MHz, so 20 MHz is adequate.
Triggering is done with a TD3A 4.7 mA tunnel diode. The trigger input is amplified and buffered by a 6DJ8-based differential amplifier driving a common-emitter 2N964 germanium PNP transistor whose collector current switches the tunnel diode.
The 283 Real Time adapter enables real-time sweeps with a 3B4.