Also applies to Unicomp Mighty Mouse Num Pad
The IBM Space Saver Numeric Keypad and later Unicomp Mighty Mouse Num Pad were low-profile desktop peripherals originally introduced in 1992 and produced until 2010. These were the marketing names of the Models M4 and M4-1 buckling rubber sleeves numeric keypads, which were available as options for an M4 or M4-1 keyboard. Like their host keyboards, the M4 family numeric keypad is a repurposed IBM Personal System/2 L40SX design. This time, the Model M3 numeric keypad option for the L40SX was used as the basis for these devices.
More information: IBM Model M3 Numeric Keypad for IBM PS/2 L40SX
The Model M4 family numeric keypad shares the same internal assembly design as the M3 numeric keypad that was available as an option for the IBM PS/2 L40SX portable computer3 that was originally released on the 26th March 19914. As such, they share a history. Development for the L40SX started in February 1990, when IBM was already behind in the 386 laptop market. Looking to cut down development time for the machine, the original buckling sleeves keyboard design skipped the usual mock-up and prototype stages and was quickly developed and pushed to production. IBM Information Products Corporation in Lexington, Kentucky under CEO Tom Hancock was responsible for the design5. Presumably, IBM decided to make desktop versions of its keyboards and keypads after the L40SX received good press for its keyboard6. The earliest year of manufacture for an M4 observed is 19927, at which point IBM Information Products Corporation had been spun off to form Lexmark International8.
The rest of the M4 family numeric keypad's history follows that of its host keyboards. Lexmark produced all M4s between 1992 and sometime between Q4 1995 and Q2 1996 and even marketed their own branded versions. In April 1996, Lexmark exited the keyboard market and all immediate production was transferred to Key Tronic. In 1998, Unicomp subsequently took over production and by the turn of the millennium had introduced their own branded version called the Mighty Mouse Numeric Keypad9. Everything relating to the Unicomp Mighty Mouse family was retired by 201010.
More information: IBM Model M4 & M4-1 Space Saver Keyboard
The Model M4 family numeric keypad was available as an official companion for host M4 or M4-1 keyboards. As such, most of the external design attributes of the host keyboard are carried over to the keypad design. The keypads are designed to match the side profile of the keyboards in all conditions, thus they also take on the familiar wedge shape characteristic to many Model M keyboards and they sport the same typically-rubberised flip-out feet as the host keyboards. The materials, key-switch technology and keycap quality are all the same as well. To summarise, these keypads are made from a PC + ABS plastic blend and have PBT keycaps that for the off-white version of the keypad has high-quality dye-sublimated legends.
On the back, the keypad has a single ethernet-like jack. It's in fact a modular 10P10C (aka, the same jack as RJ-50 rather than RJ-45) and is used for connecting a double-ended cable connecting to the host keyboard. The cable simply extends the keypad's membrane traces to the host keyboard, which in turn acts as the keypad's controller. Thus the keypad alone cannot be used by itself without constructing a custom controller for it. This lack of an internal controller is the main difference in design between this keypad and the M3 numeric keypad it's based on.
The internal PCB it does have simply transfers the membrane flex cable traces into modular 10P10C pins. The membrane connectors are 2.54mm pitch TRIOMATE sockets just like most Model Ms that have such sockets. The internal keypad assembly is similar to most other Model Ms - three layers (the barrel plate, membrane and metal backplate) that are held together with melted plastic rivets. The use of these rivets is the single biggest structural difference between M4 family keyboards and keypads since the keyboards themselves use a more robust metal 'tooth' riveting instead. As stated before, this assembly design is borrowed from the M3.
17-key Model M4/M4-1 numeric keypads use the same layout as their M3 cousins. It's the standard numeric keypad layout that's still in use today.
18-key Model M4/M4-1 numeric keypads typically gain the extra key from splitting the 2-unit 0 key into two 1-unit keys "0" and "00". The addition of a "00" key is useful for adding machines and cash registers11.
19-key Model M4/M4-1 numeric keypads also split the 2-unit 0 key into two 1-unit keys "0" and "00", but they also gain another key from splitting the 2-unit + key. The functionality is subject to variance for their right-most column. Presently, two distinct variations are known; 1u -, 1u +, 1u Enter and 2u Tab (as pictured), or 1u -, 1u +, 1u End and 2u Enter.