1973 26" Grundig 5011 "Super-Color"
I had one of these sets that i rescued from a skip along with a Philips G24T306 brought home in my avenger estate.
That diagnostic adaptor basically just shows you which supplies are working/available and a brief description of the part of the set they supply. The clue is the + 'letter', eg, +B, +C etc.... Grundig always named the main psu and lopt derived supply lines in this way.
An early ITT set (CVC32?) had the LEDs on the motherboard so you could see what supplies were present or indeed missing.