This is simple housing for a Siemens Sirius ACT EStop that will fit on 4040 VSlot profiles on the Ender 3 and be secured by the sheet metal control board compartment. The two rectangular cutouts on the back cover are made to fit two WAGO WINSTA 890-702 sockets. The back cover is held on by two M5 bolts.
I personally am using it to switch after the power supply as the (meanwell) PSU will have enough power left in its capacitors to run the printer for about a second after the power is cut.
The author marked this model as their own original creation.