This unit is a GPS Timing Mechanism made by Navstar Systems in 1990. It was developed by a research and development team working in Daventry, UK for use in the XR4 Marine Navigator GPS Receiver.
GPS receivers have to measure Time and Frequency very accurately.
To get a 1 meter accurate range, the signal time of flight has to be measured to a few nansoseconds (billionths of a second). The speed of light is 1 foot (30cm) per nanosecond. To do this, GPS receivers need to have an extremely stable local oscillator (ie clock) to measure the signal time of reception. However, the stability of quartz crystal oscillators can change very quickly with temperature.

Previous generation GPS devices tended to use Ovenised Oscillators which work by raising the temperature to a fixed, but high temperature and in so doing keep the oscillator output frequency stable. The problem with these is that they are big, power hungry and expensive. For the civilian consumer market Navstar Systems developed a cheaper and less power hungry system, including a software compensated reference XO (crystal oscillator).

In order to minimise the temperature effect on the XO, it was first surrounded by a block of aluminium to create a thermal mass. This was then surrounded by a polystyrene insulation layer and then protected inside a metal can. This ensured that any external temperature changes only affected the quartz crystal very slowly.

As part of the manufacturing process, every device was temperature cycled in an oven to calibrate the oscillator frequency vs temperature which was then remembered in the XR4 receiver as a calibration table. During operation, the XR4 software would then update its XO calibration table using the Clock Frequency output from its real-time GPS computations.