I noticed the other day that my 110.5 National corkscrew has very sharp angular threads similar to that of the Pennycuick threading style. It also has a dot on the underside of the dome.

Joe Maurath Jr. wrote in his article "A History of New England Insulators" that "Evidence exists that some, and probably all, National insulators were produced at the American Iron Glass Pipe and Plate Company (abbreviated as the "Iron Glass Company") in Haverhill, Massachusetts." He goes on to mention that "it was stated within a newspaper article during January 1885 that the Iron Glass Company was producing insulators for undisclosed companies who held valuable patents."

Another observation I had was that the American Insulator Company accused the Electrical Glass Corporation of patent infringement during the year 1890 as can be seen here: http://reference.insulators.info/publications/view/?id=12762&h0=pennycuick Could this have something to do with their disappearing act after only six months in business?

Any thoughts?

Answered. Different type mandrels/processes used while forming threads. Thanks ICON.