This is a rear mount for a BL-Touch style Sensor for the ANET A8 to be mounted on the back of the lower bearing block of the X-Axis.
For mountig with M4 screws it's required to tap M4 threads into the at least the lower two holes of the bearing block.

The Sensor offset to be set in the Marlin Firmware should be
define X_PROBE_OFFSET_FROM_EXTRUDER 0
as it is in Line with the Nozzle for Y you have to measure the distance between the nozzle and the sensor tip and add it as positive value to
define Y_PROBE_OFFSET_FROM_EXTRUDER