Appears to be the lens for an old fiber-optics light system. Looks like a FiberStars Broadcast lens, but any of their lenses will fit the opening. Most of the time the threads were proprietary, but some were just 1.5" FIP.PN1,
I have no experience with that style of light, so not sure how it seals. On my pool light, the light niche is flooded, and the seal is in the back where the wires go into the conduit.
Let's see if @1poolman1 has any inputs, or what some of our other members have to say.
Thanks,
Jim R.
Gasket, that's the orange ring in the picture.@1poolman1
Do you have any idea how they are sealed to prevent water leaking out of the spa?
Thanks,
Jim R.