was installed. Is it correct to assume that the results were destructive pre-ignition? We used to see a lot of that sort back in around 1967-68 when people would put hotter plugs in early Porsche 911s for city driving and then not change them before they took off across the country at western Interstate speeds.

The platinum plugs that Porsche recommended in those days were very different from the present day type in one respect: they could only tolerate a gap of .012"-.014" but worked great at those settings -- it was weird to be setting plug gaps narrower than point gaps, but that's the way they worked.