There is a chip on the head, see photo 5. Trying to show up the end where the drill bit goes. The end bit slides across (it's stiff to move). It slide across and moves the little "nib" that you. Can just see up the end.
The "nib" for want of a better. Word locates into the notch on the drill bit, to hold it in. The item "Wm Marples Ultimatum drill brace. Collectable woodworking tools, Antique tools" is in sale since Sunday, May 31, 2020.
This item is in the category "Collectables\Tools & Collectable Hardware\Collectable Tools\Carpentry & Woodworking Tools\Drills, Braces & Bits". The seller is "huntsell" and is located in Huntingdon. This item can be shipped to United Kingdom, Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Australia, United States, Bahrain, Canada, Japan, New Zealand, China, Israel, Hong Kong, Norway, Indonesia, Malaysia, Mexico, Singapore, South Korea, Switzerland, Taiwan, Thailand, Bangladesh, Belize, Bolivia, Brunei darussalam, Dominica, Egypt, Guernsey, Gibraltar, Guadeloupe, Grenada, French guiana, Iceland, Jersey, Cambodia, Saint kitts and nevis, Liechtenstein, Macao, Monaco, Martinique, Nicaragua, Pakistan, Paraguay, Turks and caicos islands, Aruba, Saudi arabia, United arab emirates, Chile, Bahamas, Costa rica, Guatemala, Honduras, Jamaica, Kuwait, Panama, Philippines, Qatar, Trinidad and tobago, Uruguay.