Original ceramic cap for Adjust a Wings reflectors.
You can buy it as a replacement if the one you had in your reflector is broken or damaged, or it is also used for when we have the Adjust a Wings large size. So you can put two bulbs in the same reflector, taking advantage of its size.

Valid for E - 40 thread, which is the one provided by the 250, 400, 600 and 1000 W high pressure sodium and metal halide bulbs used in indoor crops and greenhouses.