IBM Model M
Buckling Spring Keyboard
IBM released the 'Model M' keyboard in 1985, which popularized the standard ANSI keyboard layout present on all computers today (something that is often overlooked and taken for granted). It uses the same buckling spring mechanism as present on the Model F keyboards, but instead uses a rubber membrane behind the spring switches (essentially it works like a standard rubber membrane type keyboard, with a more elaborate switch stacked on top). The 'M' presumably stands for Membrane.
Later in the 90's IBM had moved Model M production to Lexmark and Maxiswitch (however, the Greenock factories still operated and produced Model Ms and other IBM equipment in lieu of that). Later the original group behind the Model M bought the rights from Lexmark and now manufacture these keyboards under Unicomp.
If you need parts for your Model M, Unicomp is definitely the place to go-- and I would advise supporting them as they're a great company: Unicomp's Website
--> 8228 User's Guide
--> Extended Character Set with Space Saver Keyboard (SSK User Guide)
Model M Variants
There have been a few variants of Model M keyboards produced, here's a list with the varying models, as well as the different colours corresponding models came in:
Model M (standard 101 key release) [PW, IG]
Model M Space Saving Keyboard (reduced to 84 keys with an integrated numpad) [PW, IG]
Model M2 (uses a lower profile key cap and redesigned barrel) [PW]
Model M 5-1 (trackball above the arrow keys) [PW]
Model M 5-2 (trackball above the LED indicators) [PW]
Model M 13 (trackpoint embedded between the GHB keys) [PW, IG, B]
Model M 15 (ergonomic split keyboard based on the M2 low profile keys) [PW]
[Colour Legend: PW = Pearl White, IG = Industrial Grey, B = Black]
I'm excluding the Model M4-1 keyboards from this list as they don't use buckling springs but rubber cups: similar to the keyboards on the ThinkPad 701C and used on the L40 SX. These were first paired with the PS/2E upon release.
The Model M2 keyboards featured tantalum capacitors which prematurely short out after awhile. Replacing these capacitors with quality electrolytic counterparts will allow the keyboards to resume regular operation.
Differences Between Models
The are some differences in feeling and sound in earlier model Ms, and different Model M types. The most notable is that earlier Model Ms have a fair amount of sound from the springs; whereas later ones are dampened to prevent the resonant "spring noise". Earlier buckling spring keyboards in general, will have bolder printing.
The majority of Model M keyboards (excluding the later ones with integrated cables) will use a jack called SDL, developed by AMP (presently known as TE Connectivity today). This was presumably designed for the fact the PS/2 interface was upcoming in 1987: allowing the cables to easily be switched from AT to PS/2 without the need of any bulky adapters. It also allowed superior shielding and easily changing the cable out.