Thanks to Terry and to "Herc-u-lease" - I am pretty sure now, the test set is for the
Louis Newmark Series 6000 Attitude and Heading Reference System. Here is a picture
of the early Hawk's AHRS panel in comparison to the relevant section of the test set -
they look pretty similar:
Hawk:
Test set:
So I assume, the test set plugs between the black box and the aircraft on the
black box side (plugs are bigger than for example the HSI's) and I will try to get
hands on the AHRS hardware via eBay in the next months... I will post updates
here later - thanks to all for the help!