The Model M5-1 is one of two variants of the Enhanced Keyboard featuring an integrated trackball pointing device attachment, along with the M5-2. The M5-1 uses a 16mm trackball placed around the arrow keys and features four mouse buttons; two for left and right click and two for left and right stepped-click. The M5-1 is the rarer of the two due to its withdrawal from production not long after its introduction. It is presently believed that the M5-1 was withdrawn due to production complexities with integrating the trackball module on top of the keyboard assembly. Collectively, the M5s were the last introduced Model M variants to feature a modular SDL connection.