Armourlight 7 inch glass suspension with clevis. The "PB" on the cap is probably Pilkington Brothers Ltd, the company that had the Armourlight trade mark.

History of Toughened Glass Before 1935, glass insulators were made of annealed glass. This type of glass did not have the mechanical-electrical strength needed for high-voltage applications. In 1935, the Pilkington Company introduced toughened glass suspension insulators (also known as cap-and-pin insulators and disk insulators) in the United Kingdom, and their use became common in the early 1950s. The fabrication of toughened glass insulators has evolved over the years in terms of the materials used and the processes followed. Today, toughened glass suspension insulators are applied worldwide at all voltage levels up to 800 kV and on ac and dc networks alike.