Western Electric 724B fixed-tuned TR Cell for use with an external cavity. It has temperature compensation and replaced the 724A. 8.6-9.7GHz. It was used in the AN/APS-1, AN/APS-4, RT-10/APS-3 and AN/MPG-1.

The device measures 55x12mm (10mm across the glass), with 10mm between the disks.

Markings on the side of the box not in the picture are:
U.S.NAVY
CONT. RXSA 23502
MAR. 1944