I hadn’t been back on this and I apologize.
Matisyohu28, not one of the rabbonim you mentioned were either born in America or Modern Orthodox. With maybe the exception of Satmar they had fairly small communities at that time.
The Modern Orthodox rabbinate of America prior to the arrival of the European ‘gedolim’ were not nearly so learned, its true, but were trained in providing the basic ministerial services of the role, e.g. visiting the sick, providing for basic communal functions and a basic Shabbat drasha. YU already existed for well over 50 years and I probably am insulting the memory of many of their graduates of that period by assuming they knew so little as that and JTS was either still producing or still had around people they had trained who were Orthodox who also were not fools.
Now in the 1950’s the haredi were a present and growing group but they were certainly the minority. You assume that because the MO had lesser known rabbonim they were the smaller group or comparable in size. It simply is not true. The MO were the larger body of people. And I agree the MO of then was different then MO is today.
As for the statement that American hasidim in the 40’s and 50’s often dressed like any other person just like the Modern Orthodox I heard that from a rabbi who was hasidic; Square if you must know who said he met these people himself.
Ultra-Orthodox is a term dating back to the generation when the haredi were a fringe of the Orthodox community, i.e. around WWII. At that time the modern orthodox were the norm and even many of the hasidim of that time dressed up like the modern orthodox.