I guess changing handedness wasn’t universal. My FIL was a lefty and he was born in 1923. I wonder if some places were not doing that, or some kids just ignored them (he was pretty feisty) or if it was regional?
Since he was born in 1923, post-WWI, your FIL would have been learning to write around 1929, when, I would think, educators had become more enlightened about handedness. My father would have started school around 1917 with teachers of a different world view.