This is a Jeffery-Dewitt link type 11.25 inch blue porcelain suspension. Dark blue '40' Ink stamp located inside the top spyder. This is an example of a design where the metal top bracket (spyder) is the same as the lower unit and discs are joined together by a link, a seperate piece of hardware.

Jimmy Burns speculates that a JD piece without any marking but has Kenova blue numbers under tine is an early Kenova product before 1920 patent. 1919-early 1920

The design of the J-D disk insulator has aimed to produce a theoretically correct insulator as practical as possible. The marked distinguishing feature of the new unit is its great thickness of dielectric and the perfectly balanced field of stress due to the identical metal electrodes above and below. The minimum effective dielectric thickness of these new disks is 2.25 inches, or about three times this amount for the single-piece units now in use. The stress on the porcelain under all operating conditions is correspondingly lower than on the thinner types. Authorities have proved that insulating materials gradually fail under stresses which cause corona. The critical stress varies with the specific inductive capacity of the material; for porcelain, the safe potential gradient is about 40,000 volts per inch of thickness. The J-D disk unit, as proved by tests, both on normal and high frequency, avoids corona up to 90,000 and even 110,000 volts, whereas a disk 0.75 inch thick is under corona at 30,000 volts. The manufacturer believes the new unit will insulate permanently, because of its safe dielectric stresses. The perfectly balanced field is also very important in securing the full value of the insulating material and enabling the insulator to resist high frequency and other line conditions caused by lightning and switching. In the cap and pin type the area of contact with cement outside is six or seven times that in the pin hole, causing a greatly concentrated stress on the porcelain next the pin hole . Tthe Insulator Gazette has several more articles on this company .