This is a replacement part for the SE-6609 Photoelectric Effect Apparatus. It is the photodiode tube connected to a metal panel that replaces the back panel on the photodiode enclosure. In other words, to replace the photodiode tube, you buy SE-6612 and replace the tube and the back panel at the same time (the tube is soldered to the back panel).
CAUTION: UV LIGHT
Avoid eye & skin exposure. Wear proper eye & skin protection.