The Tektronix 321 is a very compact single-trace, single-timebase portable oscilloscope that is almost completely solid state.
It was introduced on April 8th, 1960.
The 321 includes several germanium bipolar transistors, which are a known reliability problem.
The 321A was introduced in 1964. The 321 and 321A have slightly different circuits.
Early instances of the 321 (sn pre-720) use a 5718 peanut tube as the front-end cathode follower.
Later 321 and 321A use a 7586 Nuvistor for that role. Latest 321A (SN6000 up) use a FET 151-1011-01.
In several circuits, the 321 uses OC170 transistors where the 321A uses 2N2207 transistors.