New Old Stock - Vintage Mason Master concrete/brick drilling convertor with chuck key and 2 masonry drills- no 10 & 12 - [approx - 6 and 7.5mm] the chuck is 3/8'' -[10mm max capacity] and is designed to fit into an ordinary drill -[ 3/8'' or larger chuck] that doesn't have hammer action and then enables you to drill bricks/concrete, the packaging is a bit shabby but it is over approx 40 years old.