The Model M came with the IBM PC/AT, years after the original PC and subsequent PC/XT. Those came with the Model F keyboard which had a terrible layout.
The Model F sucked. The key mechanism was fantastic, but the layout was utter trash. The weird return key that's long vertically but has a one-key-sized raised section in the middle is the worst part of it.
You can buy brand-new keyboards with this mechanism now from some small business (sorry, don't have a link handy), but even they offer Model M-like layouts because the original Model F layout is so awful.
The only good thing I can say about the Model F layout is that almost all microcomputers at that time had terrible keyboards, though the reasons they were terrible varied. Compared to junk like the Atari computer keyboards, it probably seemed great, though of course the IBM PC was far more expensive. For really great keyboards, at that time, you had to look at the business-level terminal keyboards and such.