I think that the answer is SPLINE. Start with SPINE, which, according to my dictionary (Bloomsbury Concise English 2nd edn.) can mean, "sharp point on plant, e.g. holly." or "quill of a porcupine." So, a SPINE can be a longish, pointed article, as can the tang of a file. The tang of a file is the longish, pointed part that fits into the wooden handle. Take SPINE and insert L (for Left), which gives us SPLINE. A spline can be a flat, narrow piece of metal, called a key, that locks a gear wheel on a shaft. It looks rather like a floorboard nail without a point. It can also be a thin, narrow piece of wood that is used to join two boards. A SPLINE can thus be a longish, pointed (sort of) article. So, we start with SPINE (a longish, pointed article, as is a TANG); insert L; and we get SPLINE (a longish, pointed article, as is a TANG. I know that I've gone on a bit, but I think that a knowledge of mechanical engineering is useful here. I hope that this has helped.