A bit with a removable center through which surveying instruments were run into the open hole. Used prior to industry's general acceptance of non-magnetic drill collars. (Bul D20)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}