Size of a heating programmer back plate retaining screw

[jacoscar]

I have a heating programmer which is missing the square nut where you put your screw in to lock the front in place.
I still have the screw, its diameter including the thread is 3.4mm; I’m not sure if it’s an M3 or M4 or maybe an imperial size… does anyone know? I need to buy the correct square nut

 

[flameport]

The only metric thread close to that is M3.5, as used in electrical backboxes.

 

[jacoscar]

Thank you JW