I would not recommend pressure switches at all. They are very prone to failure.
The M3 is designed to be easily toggled on and off via a paddle switch on the back of the unit that does not force you to change your hand position significantly. The paddle switch allows you to do momentary on or constant on.
You definitely do not need the pressure switch.
I have the same unit for sale, but also including a rechargeable battery set AND an extra 3 cell bulb that outputs more light power than the real M3.