LFrank, I know you asked RM, but...
If I had reason or just curiousity to check one plug, I would check them all. Each cylinder can and does have its own characteristics. So, while by the luck of the draw, the single plug you check might be fine, the very next in line could show signs mal-peformance.... lean, rich, oil control, etc..
My 2 cents.
And I would also like to extend my thanks to RM... that pdf is a ton better than the illustration I've been using in my many Haynes manuals.
`68 280sl
`62 Lincoln Continental Conv.
`73 450sl