The 724B is a TR cell, a device used to protect the receiver portion of a microwave transceiver which shares the same antenna for both transmit and receive functions. A TR cell functions like a triggered gap tube, and blocks the receiver hardware by shorting the receiver input during transmitter operation. Transmitter pulses cause the spark gap to break down, shorting the receiver input. The 724B is filled with water vapor, which produces a blue glow when ionized by a high voltage field.
The 724B was used in several WWII-era 10 GHz mobile radar systems, including the AN/APS-1, ASD (AN/APS-3) ASH (AN/APS-4) and AN/MPG-1.
The spark gap connects to the copper external rings. These would be clamped across the waveguide. The top connection would supply a high voltage from a high resistance source to maintain the gap just below ionisation. The RF pulse would then ionise the water vapour very quickly.
The end connection and copper rings.
The thin glass tube envelope is 9 mm in diameter, and including the base is 55 mm tall.