You are right, how could I have forgotten the little R mount? I hang my head in shame....
Having said that, as you point out, history, as evidenced by the lenses available at the time, doesn't actually support the problematic 50mm theory, and it is worth noting the R mount had the same 42mm flange distance the FL mount did, and that was the same as the FD and FDn.
I'd hardly lump the 1950's into "modern lenses" but each to their own.
But back to more interesting stuff, didn't Olympus push for shorter "standard" lenses in the '70's, I remember seeing kits with the 40mm and OM-? Though even they had a proliferation of 50's from a 1.2 to a 3.5 macro and the one 55mm 1.2.