A forum member over on Deskthority recently asked IBM and they responded!  Great detective work.

An IBMer researched the company archives and noted that IBM’s overall keyboard designs were sequentially labeled by letters.  The Model F was referred to as the “Keyboard F” initially.  The older keyboard designs were named with earlier letters.

“Keyboard F Mechanism Manufacturing is doing more with less versus previous keyboards, Specifically the IBM Model B keyboard mechanism and the Model E keyboard mechanism.

It would appear that Model F and Model M are sequential models of keyboard mechanism.

The article mentions that Model B keyboards have an absurd 9 moving parts, where Model F only has 3. I assume the cap, spring and flipper.

So it is actually pretty likely that “M” doesn’t reference Membrane at all.”