That's a question I have never thought about. What I do know is most all implements from the 40's and before had square heads, and the ones from the later fifties tended to incorporate both. Both my 44H series had square. Maybe someone with a 444 plow could tell us if it had square or hex. I would think Hex heads had to wait for grade 3 steel hardware to be affordable, or torque pressures to be greater than the softer bolts could stand. Dad bought a new 1948 Cat D6 which was all hex grade 3 or better. I remember old Johnny saying, "that's a Caterpillar bolt. Put that in there, it'll stay" The square ones were of softer steel, easily rounded off with an ill-fitting wrench, so hex heads of this steel would have been Really easily rounded off. They probably did it gradually, not with a boom. If I were a family farmer back then with a toolbox full of square-ended wrenches and went to buy a plow (or anything else) and found out I would have to buy a whole new set of those "new-fangled" "six-sided" wrenches along with it, then I might choose not to. Remember, all the tractor companies were still coaxing farmers into retiring their mules in 1950. Ever seen a Monkey Wrench? Ever try to turn a hex with one?
The real boom in ag machinery can about be time-lined to about 1960, when the WWII vets came to maturity. Their fathers (my grandfather)farmed during the Depression and still held the purse-strings of most farms until they started reaching retirement age during the late 50's-early 60's. They remembered the Hoover Days all to clearly and were none-too-eager to spend money unnecessarily. The 1950's was a transition period where mule-driven technology and its malleable iron slowly gave way to horsepower and cold steel.
So... after a whole lot of rambling... I don't have a clue.