This is a replacement, spare stem screw for the Minelab Vanquish metal detectors, used to fasten the middle stem to the control box and the arm cup to the upper stem. The screw is M4 thread and 10mm in length with a button head, fitting a 2.5mm allen key. Made from stainless steel to resist corrosion.
Use this spare as a replacement for a lost screw. You may want to use some thread lock adhesive to prevent the replacement working loose in future.