Radio Bearing Indicator, ID-307/ARN for AN/ARN-21 TACAN

The Radio Bearing Indicator, ID-307/ARN was a component of the AN/ARN-21 TACAN system used in US military aircraft of the 1950's thru 1970's.  

TACAN stands for Tactical Air Navigation, a system that allows pilots to see the distance and direction to a ground beacon, usually at an airfield or aircraft carrier. This equipment transmits interrogation pulses, receives beacon pulses from the TACAN surface beacon and also prepares the received information for display on the bearing and distance indicators.  The ARN-21 receives in the band 962 to1213 MHz and transmits between 1025 and 1150 MHz on any of 126 switch selectable channels. 

According to Designation-Systems.net., AN/ARN-21 was used in across many US Air Force and US Navy aircraft, such as:

A-3A/B, A-4, B-50, B-52, C-54, C-97, RC-121, C-130, KC-135, C-141, F-84, F-86, F-89, F-100, F-101, F/TF-102, F-104G, CH-3C, HH-43B, T-29, T-33, T-37, T-39, OV-1A, X-21, AF-2W, AD-5, F2H-2/2N/2P, F3D-2, F9F-2, P2V-5, R6D-1, just to name a few!

The pointer indicates the bearing of the ground station, in degrees from the aircraft's current course. E.g., the indicator in the photo shows that the ground station is 108 degrees from its current course.

Measures 3.25 inches diameter and ~6 inches deep. It is in fine condition given its age.