Rhode and Schwarz R8401 test set ex RAF / Luftwaffe surplus teardown. What was this used for?

I got this interesting looking box from a military surplus auction, which caught my interest as it has some useful parts for DIY test equipment and also the case would be re-usable if the front panels were replaced. It came with an instruction manual which contained the service manual complete with schematics. However it was all in German as is the front panel text.

Armed with Google Translate I determined it to be some sort of constant voltage source and measured current from whatever it was testing. I really have no idea and this thing was clearly a one-off or very limited production run specially made for the RAF and likely the Luftwaffe as well. Perhaps it has something to do with the Tornado as this aircraft was used by both air forces.

I take a look at the manual and take it apart. It was very difficult to get any salvageable parts from this, but I managed to get a few transformers and three 5 digit panel meters. Bit of a disappointing teardown, I expected a bit more inside it.

If anyone knows what this thing is (or more accurately, was) please let me know in the comments.

Oops. I forgot to scan that manual before I threw it out when I scrapped the unit for spares

00:00 intro 00:30 a look at the unit and user manual 05:52 teardown - how does it come apart? 07:45 power up test 08:30 teardown in full this time ... https://www.youtube.com/watch?v=DtXJspya1NQ