if present, says "M2" rather than "M". M2s were poorly fabricated and notoriously unreliable; comparatively few survived into the 21st century, and Model M enthusiasts do not value them. The M2 should in turn be distinguished from the M5, another Lexmark variant which returned to the rugged Model M case/backplate construction but added a built-in trackball; and those two from the M13, which was also built like rugged Model Ms but featured a pointing stick. The M5 and M13 designs are still carried by Unicomp in 2021 under the names Trackball Classic and Endura Pro. Design The variant most commonly referred to as "Model M" is Part No. 1391401, on which many other variants were based. This model, known as the Enhanced Keyboard, included IBM's patented buckling spring design and swappable keycaps. The Model M's design has been widely praised as durable and reliable, and has remained basically the same since the 1980s, while virtually all other computing hardware, from PCs to monitors to mice, has changed dramatically. The M's design, including its heavy steel backplate and strong plastic frame, has allowed many early Model M Keyboards to still be functional with no modification nearly four decades later. The Model M's buckling spring key design gives it a unique feel and sound. Unlike more common dome switch designs, buckling springs give users a notable tactile and auditory feedback. Because of its more defined touch, some users report they can type faster and more accurately on the Model M than on other keyboards. Additionally, many model M enthusiasts believe that tactile-feedback keyboards like the model M reduce stress on the hands, preventing or even reversing Repetitive Strain Injury. The key presses require strong pressure and a pronounced sound, meant to help typists previously trained on typewriters who had become accustomed to that level of feedback. This resulted in the Model M sometimes being referred to as the "clicky keyboard." Unlike competitors such as Cherry and Alps-style keyswitches, buckling-spring keyswitches do not have a plunger part with sliding surfaces that can be fouled by contaminants. This makes Model Ms much more resistant to dirt, dust, and grit, and is a significant factor in their long service lifetimes and appeal in challenging industrial environments. Until the late fourth-generation variants, most Model Ms were manufactured with a 1.25" slotted, circular speaker grille in their bottom surfaces. Relatively few contain an actual speaker, however, which