The Tektronix 067-0508-00 Amplitude Calibrator generates a square-wave with two precisely controlled voltages and 50 Ω impedance at both voltages.
This could be achieved by a 50 Ω resistor in series with the output of an amplifier with 0 Ω output resistance (e.g., an opamp with plenty of negative feedback). But that is not how Tektronix did it. They used a 50 Ω shunt resistance at the output, and a precision DC current source that can be steered toward the output or toward a dummy current sink transistor. The built-in multivibrator controls the steering of the current.