layout keyboard bundled with the IBM Personal System/2. Until around 1993, most Model Ms included a sturdy, coiled, detachable cable, with either an AT (pre-1987) or PS/2 connector, in 5- and 10-foot lengths (1.5 and 3 metres). From about 1994 onwards, flat non-detachable cables were used to reduce manufacturing costs; however, IBM retained its 101-key layout, never implementing the Microsoft Windows keys common on other keyboards from that time. Unicomp later designed a 104-key Model M with Windows keys. On March 27, 1991, IBM divested a number of its hardware manufacturing operations, including keyboard production, forming Lexmark International. Lexmark continued manufacturing Model M keyboards in the United States, United Kingdom, and Mexico, with IBM as their major customer. Many of these keyboards are identified by IBM assembly part numbers 52G9658, 52G9700, 71G4644, 82G2383, and 42H1292, which were bundled with IBM PS/ValuePoint and IBM PC Series. Over the next four years, cost pressure led to several minor design changes intended to lower the part and fabrication costs of Lexmark Model Ms. The case and metal backplate were repeatedly lightened. The cable jack and detachable SDL cable were replaced with a fixed cable. Some variants were made with a single color for key legends. In 1995 Lexmark made the most sweeping design change in the Model M's history, altering the size and location of the internal controller board. While the new "press-fit" design successfully lowered manufacturing costs by eliminating the two ribbon cables and separate LED daughterboard of older versions, the controller's new card-edge connector proved to be a failure point that shortened the keyboard's average lifetime. The classic era of the Model M is generally considered to have ended with this change, though a few on the older pattern continued to be made at Greenock and Guadalajara until 1999. Relatively few press-fit model Ms have survived. During the Lexmark years, a few Model M variants were manufactured with rubber-dome keyswitches rather than buckling springs. Due to these switches' comparatively short durability, few of these variants have survived. Despite their rarity, today's enthusiasts and collectors do not value them nearly as highly as the more common buckling-spring variants. A five-year agreement obligating IBM to purchase nearly all of its keyboards from Lexmark expired on March 27, 1996. Lexmark exited the keyboard business, selling related assets to IBM and Maxi Switch. When Lexmark discontinued keyboard production in April 1996, IBM continued producing buckling-spring