Just a footnote to this discussions on sparkplugs. I was out rooting through one of my trailer, looking in boxes I put away 25 years ago and I found some more, radial/aviation style engine sparkplugs. The first another Champion C63, but with a twist. check out the twin electrodes. It the first one I remember seeing.
Second I found a couple of cartons of plugs for the Light Tank (Locust with Lycoming air cooled aviation engine) which are also Champion plugs. Note they are single electrode and unshielded.
Here are a few pictures.
