Knock-In Injector.
Used to resize an injector to a new size. Knock-In injectors have a 4mm diameter shaft which can be tapped in with a rubber mallet into an exisiting injector that has been drilled out. One tapped in, they can be drilled from behind to the size required.
Gameco stocks a wide range of Orifice/Gauging Drills to suit whatever size the injector requires.