Optical Sensor, PN 1235373, used with Logix Electronic Metered Control ( all use this part ).
And Autotrol 360, 368, and some Pentair ( Fleck ) = module,sensor,photo interrupterCritical part
It is used to "tell" the control board "where" the Camshaft has moved to, in the various cycles.
Failure ( or just being dirty or wet ) can cause Err 3 in many Logix controlled valves.
Reference = Autotrol Logix Err 3
To see how it's installed, see this upgrade video, Logix Upgrade Video
Also see this video, Autotrol 268 Optical Sensor
Also used in Fleck ( Pentair ) model 5800 Series of valves ( 5800, 5810, 5812 ).