The ORP90 is a Cadmium sulphide light dependant resistor. See also ORP11. Unlike the ORP11 the ORP90 has a large light gathering surface thus adding to its sensitivity.
The design use was for light level based switching applications.
The active material is seen as dark. The substrate is showing in an interdigital pattern. This gives increased length to the resistive material.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 52 mm tall.
Reference: Datasheet. Type ORP90 was first introduced in 1958.