I understand both KLG and Champion spark plug boots/caps were used. I'm a thinking that supply may have been short, same thing happened with wheel/pivot bearings with some being metric. Got to keep building the bikes and use what's available ?
Absolutely and that is what originality hunters forget. Just imagine this situation at the Vincent factory.
Sir we have run out of Champion plug caps do we:
1. Stop production, send the workforce home until some more Champion plug caps can be sourced.
2. Go and buy some KLG caps (or other make) to keep production of bikes going.
This would have been the same situation for anything that is a general consumable like spark plugs, handlebar levers, batteries, chains etc. As long as it fitted and was to specification.
Simon