[Why did Robert Plant hate David Covedale?]

I'm no Deep Purple expert - but are you sure about this? Gillan came in during 1969 - Zeppelin weren't THAT well known in the UK at that time, and just starting to get a profile in the US?

The quote was definitely in reference to Gillan - Zep's first album had come out and was selling well so the buzz was already beginning to spread.

It's also possible that the members of Purple Mk I had somehow crossed paths with Zeppelin in the States, during the latter's early tours. Maybe Blackmore witnessed the Zep audience response first-hand and realised that was where things were headed, hence Mk II?

[Zeppelin Mysteries Hosted by Steve A. Jones]

Could the two titles in question be the working titles for songs that were later released under other names? Or the original versions of songs that were quite different in their finished (released) forms?

[Jimmy Page AUTObiography]

It must have been about three or four years ago now but he was quoted in passing that it would be fun to put it all down on paper one day, something like that. I haven't heard a peep since then so I assumed it was a throwaway quip. If it's happening, though, that would be all kinds of coolness.