Late 1937/Early 1938 Indian Spark Plug Question

[motor-officer]

I recently picked up several unique Edison "Indian C" spark plugs. The reason I say unique, is that they are about half the size of a "normal" spark plug. I was told they were used for a short time in 1937/38 Indians with a special adaptor. Can anybody tell me if this is correct and happen to have any photos of the adaptor? Any help would be appreciated.
Thank you.