NGK 16.75 inch porcelain suspension with 1.75 inch ball. I have no information on this type. A similar suspension in glass [id=274553703], again with no information available. A similar style made by Canadian Porcelain is shown here [id=268318224] .Note their size compared to other discs.

Glenn drummond recently sent a link to this page . Some history of early testing which includes a similar suspension to this, but a much earlier production. http://reference.insulators.info/publications/view/?id=3704