...For me, what is interesting is that two fundamentally different systems of riding emerged - 'Western' from the cowboy and 'English' from the cavalry. The reason for the difference can only be the horn - the soldiers did not herd cattle and therefore did not need a horn...
The horn may be part of it, but the drawings I've seen of cavalry manuals from the mid-1800s would probably look fine to QHriderKE's Dad. Even dressage riders had some points in common:
Look at the long leg, somewhat forward, hips back, feet forward in the stirrups...
Cavalry (if he would just 'home' his feet in the stirrups):
This rather famous US General:
Heels at or in front of the belt buckle would seem pretty normal, as was the long leg.
A sheriff, probably around 1920-30:
This photo has a good story to it:
""This is my Grand-dad Alex McDonald on 'Old Paint' with his son Fred on the back and a neighbor boy. About 1929-30. When he would go to work at the meat packing plant he would give Old Paint a good slap and Paint would go home so the family could use him. At the end of the day a slap would send him back to get Alex." Historical Horse - Home
And WAIT! Who is this fat guy on the little Appy?