These boots are designed to fit bat-handle toggle switches, push-button switches, and panel-seal circuit breakers.  They bond to a mounting nut to prevent delamination and offer excellent environmental protection. 

Base Thread Size 1/2-32 NS-2B (internal thread for mounting on toggle switches or waterproof circuit breakers. The material is Flexible, tear-resistant black silicone rubber (provides watertight seal, vibration resistance, and wide temperature tolerance)

Storage/Shelf Life:  Silicone lasts years but keep away from extreme heat/oils.