Dionela - connection part 1/2''
It consists of 2 pcs: a valve on cold water inlet (functional part), and a distance piece in warm water inlet. This switch has an advantages: you can filter independently on battery operation (eg, washing dishes).
Note - you will recognize the pipe size even without removal of the tap. If the pipe diameter is close to 3cm it means it is 3/4" pipe (the most common), if the pipe diameter is close to 2,5cm it means the pipe is 1/2".