Transporting acetylene tanks

I’m not a welder, and I’ve seen the discussion on transporting acetylene tanks vertical and why. However, I remember my Dad transporting his tanks laying down in his pickup back in the 70’s. He used the torch at home and on the job. He started laying them down after another welder had his vertical and was killed when he was involved in a wreck, rolled his truck, and both tanks ignited. Is the rule about how to transport the tanks something relatively new, or was my Dad pushing his luck?