This a Tektronix TM504 4-slot mainframe that accepts TM500 series test equipment modules. It is an extremely versatile platform where all types of test equipment can be plugged in. It has a 10’ power cord and a handle to carry the box around. The mainframe works and is in very good condition aside from some scratches.
This mainframe comes with four PS503A Dual Tracking Power Supply modules. They supply two 0-20V feeds at 1A and can operate independently or together (dual tracking mode). The three modules on the right hand side work and have been tested; the one on the left does not work and its pullout tab has broken off.