The BLtouch Version 3.1 - Current features a poly-carbonate plunger. This is an improvement over the V1.0 which was metal as poly-carbonate is much more durable and can be bent back into shape if deformed. 

This plunger serves as a replacement for damaged BL touch plungers. This can happen if the plunger gets caught on a loose print, or if the nozzle is buried into the bed. Ensure that you determine what happened to damage the original plunger before installing a brand new one. 

To install this plunger, turn the Allen key on the top of the BL touch until the grub screw comes out. Flip the BL touch upside down and the old plunger will come out, then insert this new one. Reinstall Allen key.