A fit so perfect, it disappears GIF

contrary to what you may believe, this is made from two separate pieces, not one

A perfect place to hide a key?

EDM, Electric Discharge Machining removes material using basically sparks, and as you see, is so accurate(40 millionths of an inch all the way down to 10 millionths of an inch) that the manufactured hole and plug mate absolutely perfectly. The machine shop guys at my company earned their keep by "burning" out the very occasional jammed drill bits or totally seized fasteners in thick, critical aircraft structure without actually touching the sides of the existing hole. F'ing magic!

That Spade is pretty slick.