Hi. I bought a 2-3" micrometer at a garage sale today. It is a Brown and Sharpe 30-1. It is missing a piece on the top of the barrel. There is a screw hole. Does anyone know what this missing piece is for and what is lost without it. The micrometer was not all that expensive. I compared it with the inside jaws of my harbor freight calipers, and they are different by .002", but that doesn't mean the micrometer is off.