3.88 MM security screw bits are specially crafted hardware tools used to open NES and Famicom game cartridges. These screws, found on the backside, have a special tamper-free security groove to prevent modification, hence the need for these little guys.

Why would you want to open one of your NES carts? Either you're a demented NES fan wanting to see the circuitry and insides just for sheer pleasure. Or, and the more practical task of usage, you are in search of a Famicom-to-NES adapter found inside a number of older carts like Gyromite. (These adapters will allow you to play import Japanese FC games on your American NES.)