Tektronix AA 501 Distortion Analyzer Plug in in Baltimore, MD, USA

Up for sale is a used Tektronix AA 501 Distortion Analyzer Plug in.
It is being sold AS-IS / For Parts due to its age, it will likely need to be serviced.
Original Manufacturer Description:
The AA 501 Distortion Analyzer provides completely automatic measurement of level, total harmonic distortion plus noise (THD+N). Automatic set level, automatic tuning, automatic nulling of the fundamental, and autoranging of the display all combine to permit completely hands-off operation once the mode is selected.
Just apply the signal of interest and read the 3 1/2-digit display. A novel analog-like bar graph simulates an analog meter to assist in peaking and nulling of applied signals.