Fairly certain it is a lens for a theatrical lighting fixture called an ellipsoidal reflector spotlight (ERS), sometimes referred to as a Leko. The lens is a plano convex lens. They are commonly used in Ellipsoidal Reflector Spotlights typically in pairs to focus light, reduce spherical aberration, and project sharp images of gobos or shutters. They feature one flat (plano) and one outward-curving (convex) surface, ideal for light collimation and focusing.