The ET0590 Diode Test Set is an undocumented HP-internal instrument that appears to be designed to trace the V-I characteristic of diodes. No information has been found about this instrument. In the photo below, the ET0590 has a 1N4004 dioded placed across its binding posts.
The second photo shows the rear of the instrument with the top/rear cover removed. On the circuit board facing the camera, three holes can be seen. These holes provide access to screwdriver-adjust variable resistors which are, from left to right, horizonal position, focus, and vertical position.
The ET0590 uses a 2BP1 2-inch electrostatic deflection CRT.