This is a Pentair 78244100 Small Stainless Steel Light Niche for Concrete/Gunite Installation in pools and spas. It has a 1/2 inch rear hub. This niche is used with SpaBrite and AquaLight Spa Lights.
The small light niches measure 6.5 inch total face diameter, 4.94 inches for body diameter and 5.55 inches in length.