I have seen lot of ampere meter shunt like this. What is the reason for this small cut?
To be accurate, a current shunt must have a specific resistance value. The method used here to get that resistance just right is to build the shunt with a too-low resistance, then trim it by cutting into the bar until the measured resistance (or rather, measured voltage output for a known current input) increases to the wanted value.
It would also be possible to build the shunt out of components with more predictable properties and assemble them more carefully (repeatably), but doing it this way is simpler because the shunt can be made out of metal bar stock of no particularly precise dimensions or composition; the trimming process, done last, removes all sources of error preceding it.
The same process is also done with smaller precision resistors as laser trimming — burning the resistive material off of the non-conductive substrate using a laser.
That is how the shunt is calibrated for accuracy as it reduces the area to pass the current.