This is a mount for auto bed levelling, using a Hall-Effect sensor.
Use a BZP (not stainless) M3x25mm cap head bolt for the plunger.
Use 4mm neodymium magnets. (buckyball cubes).
MAKE SURE THAT THE PROBE IS EXTENDED BEFORE YOU HOME/LEVEL, AS THERE IS NOTHING TO STOP IT DRIVING THE HOTEND INTO THE BUILD PLATE OTHER THAN THIS. (At least in my setup, if the original z-axis endstop was used, you could wire this in parallel as a backup, possibly.)

The part is best printed in ABS, PLA will warp over time.

Use http://ooznest.co.uk/RepRap-Parts/Hall-Effect-Endstop
The part has nut-traps for easy installation and removal.
Use buckyball cubes or other 4mm neodymium magnets.
The sensor needs bending at roughly 40 degrees. (so it is in line with the slot)
Test a magnet to find where the sensor works, by powering the sensor with +5v and gnd, and moving it near the sensor. the top edge should be the triggering edge. Mark the edge with a marker pen.
Use another magnet, and find an edge that pulls the two magnets together. Mark that edge.
Push the second magnet into the stationary square, making sure that the marked edge is at the bottom.
Drill the plunger hole to 3mm, make sure that the M3 bolt slides easily in the hole, but with as little non-linear movement as possible.
Put the sliding magnet into the slot, making sure the marked edge is at the top, facing the sensor.
The bolt is held in place by the magnet.
Make sure that the magnet and bolt slide easily and drop with gravity when held vertical.
Fix to the printer with the two holes opposite the original 40mm fan.
The firmware must be set up to use end-stops only for homing, and the z-min must be inverted.
firmware code:
const bool Z_MIN_ENDSTOP_INVERTING = true;
'#define min_software_endstops false
'#define ENABLE_AUTO_BED_LEVELING
'#define Z_RAISE_BEFORE_HOMING 2
'#define Z_RAISE_BEFORE_PROBING 15
'#define Z_RAISE_BETWEEN_PROBINGS 4
'#define X_PROBE_OFFSET_FROM_EXTRUDER +20
'#define Y_PROBE_OFFSET_FROM_EXTRUDER +0
'#define Z_PROBE_OFFSET_FROM_EXTRUDER -7.00
(remove the ' from the beginning of these, a # at the begining of a line makes the text HUGE on thingiverse.)
(these offsets are only for reference, you should go through the setup here to determine them for your printer, especially the z_probe offset, as I am using an e3d hotend, not the original v10, (v10 was 60mm, e3d v6 is 62mm, roughly, so a value of 9.00mm is more likely what you will get).
how to get the offsets, https://www.youtube.com/watch?v=YpiOsetkIRg