Actually it is a plain reed switch.
When you take the cover off to install the batteries. It is in plain sight on the Left Side of the PCB.
One screw in the center of the battery holder. Holds the PCB in the case.
You can even find the internal photos, parts lists, schematics etc. in the FCC Database.
In the Details Tab.
The DS11A was FCC ID. For the one made for Radio Shacks 49-1000 Console. RS# 49-1001 D/W Sensor
Only difference between the DS11A and the DS12A is the cover on the DS11A is held on by a screw. The DS12A snaps on.
I forgot to mention when you remove the cover. It sends a Tamper message if it has batteries in it.