This is the story and (aiming to be) definitive article on the IBM Model M4 and M4-1 Space Saver buckling sleeves keyboards, the keyboard that came before and largely inspired the first generation of ThinkPad keyboards (the Model M6).
Well… sort of. The story is a bit more complex but everything is explained inside! This has been over a month in the making and features contributions from several awesome people! All contributions are acknowledged at the end.