pretty sure that the dividing line date is there because during WWII the auto companies weren't making cars- they were making tanks, engines,planes, and trucks. Ford was making copies of Willys copy of the Bantam (jeep) - ford even had ovaled F's in the bolt heads on their jeeps.

so no new cars were being built until after the war then the first year was a slam fest to get going again- using the older tech from right before the war. while that was happening the engineers were putting together "the new stuff" with tech gleaned from the war projects+natural progression of time. So when the new stuff came out it was a big difference.

traditional hotrods have (traditionally) been based on prewar tech, but people are coming around.

if i am talking out my keister, someone please let me know.