I got these along with my mill, chucked them into a drawer and just found them a couple days ago. They look to me like types of boring head? Turning the indicator drives a worm that (presumably) moves the sleeve in and out, though the few with sleeves are very stuck. How is a tool held in these? There a cap head screw going through the bottom (one is sticking out in the picture) but it doesn't seem to do much.