I had one of these in high school in the mid 1960s. It's a Heathkit PS-3 variable-voltage regulated power supply: B+ 0 to 500V/100mA or so, heaters 6.3V/4A AC, bias 0 to -100V or so.
The switch on the left switches the meter between B+ voltage 0 to 500V and current 0 to 200mA. The potentiometer on the right adjusts the B+ output.
The two tall black cylinders on the left are the series pass tubes, 1619's. The 1619 is a first-generation RCA metal tube (note the sticking-out flange near the base), and is an original metal 6L6 with the heater-cathode replaced by a 2.5V/2A filament (so required an extra filament winding on the power transformer). The 1619's plate dissipation is rated at 15W maximum, limiting the B+ output current at low voltages.
The two tall miniature glass tubes between the 1619's and the power transformer, are 0A2 gas-filled voltage-regulator tubes, essentially 150V zener diodes. They are connected in series to give a -300V reference, and glow violet in operation.
The three tubes on the right, from front panel to rear of chassis, are: 6X5 half-wave rectifier for negative supply, 6SJ7 error/control amplifier for B+ output, and 5V4 full-wave rectifier for B+ output.
This is all from memory, so some may be NQR (not quite right).
It's most likely an antique adjustable power supply that plugs into the AC mains.
Clues: the large transformer for stepping up / down (most likely) voltage, back of analog meter, back of meter selector / power switch (left) and potentiometer for adjusting output voltage (right), two vacuum tubes for AC voltage rectification (left) + large black cylindrical capacitors for DC filtering, and additional vacuum tubes for voltage regulation (right). Additionally there probably are many discretes underneath the metal chasis. Point-to-point wiring via hand assembly was very popolar in this era before PCB manufacturing.