Sensor on B8 Manual Gearbox

[clubman22]

Hi, My 2010 2.0 TDI A4 is hitting high mileage and has a problem with the manual gearbox.  I have it with my local garage to swap in a second hand gearbox from a breakers yard.  The original gearbox (that is coming out) has a sensor/switch in the gearbox housing to the left had site of the gear lever.  The gearbox to be swapped in does not have this.  I don't have a picture unfortunately.  I cannot find out what this does and if it is important.  Does anybody have any ideas ?  

[Steve Q]

The gearbox you're swapping in should be the same part number.