The engine died because of damage that probably happened earlier. The spark plugs were just another example of things that had been done wrong.

The original engine seems to have had a cambelt breakage in it's recent history, and a very poor subsequent repair. There were 4 newer valves on the cylinders 1,2. The head faces had been very badly cleaned, leading to headgasket failure. During the stripdown, I found 2 cracks in the block. One small one between a head bolt hole and the water jacket. One bad crack in the main bearing support, from the oilway. And further damage in the bearing support when the centre main shells turned 90˚ It's all in the photos.
It wasn't apparent at the time that the wrong plugs were fitted.
It only became apparent when I was checking the plugs for the current engine, that the wrong plugs had been fitted to both engines.

85 WR quattro
90 V8 Silver, 91 V8 Black, 93 V8 Lago
93 100e quattro V6. Titan, i think