The 603 looks to be thermal delay tube designed for operation from a control voltage. The heater current has to be limited to 35 mA. The heater has a cold resistance of 1.4 kΩ, suggesting a voltage of 50 Volts.
The switch contacts are brought out to multi stranded twisted wires.
The internal metalwork is substantial.
The heater wire passes along the tube many times.
The wide glass tube envelope is 30 mm in diameter and, excluding the base wires, is 150 mm tall.
Reference: Observation. Type 603 was first introduced in 1952.