When I started this type study, I thought that the Type K5 No.2 had emerged, like Einstein's Theory of Relativity, nearly in its final form. However, there were four earlier types (K1, K2, K3 and K4) that are distinguished by a simpler mechanism for handling spindle end thrust.  The Type K5 (and all later No.2's) now has a set of tiny steel balls hidden inside the upper bearing housing of the frame. I'm sure that other hapless owners only discovered this when they took apart the spindle and pinion gear for cleaning ... The Type K5 also has a new chuck design, patented September 20, 1890, which was first introduced on the Type K4. The chuck has three flat jaws instead of the 1877 chuck's mere two, but the jaws are still loose & not spring-loaded in any way. The 1/4 inch capacity of this chuck is still rather small for its size, but it theoretically centers all round bits within its capacity. The Type K5 with its ball-bearing spindle almost certainly came after the Type K3 and its unpatented three-jaw chuck that mirrors the Lanfair Patent of 1895, yet the Type K5's chuck was patented in 1890, five years prior to the Lanfair Patent.  The K4's three-jawed, springless chuck bears a "PAT APPLIED FOR" stamp underneath the Millers Falls Co. name, and it is reeded like the K3's never patented three-jaw spring chuck.  Note the egg-shaped rosewood side handle; its shape was borrowed from the handles of the Millers Falls boring machine. My son's BIL owns a house in Newark, NJ, that has umpteen doorhandles of nearly identical design, except that Richard's handles do not have the central steel shaft to anchor the attachment screw. The Type K4 and the earlier types K1, K2 and K3 have rather fragile main gears, because the spoke that carries the load from the crank was only slightly thickened to accept the attachment screw. Remember, in the earlier types the crank was attached separately to the cross shaft on which the main gear was also pinned, so the torque from the crank was transmitted first to that shaft and then to the main gear, an even less efficient process from a strength-of-materials viewpoint - the pins tended to shear off.