When 2 x 4s were actually 2 x 4….
2x4’s were never 2” by 4”
Not in the 1960s, those days were already gone.
Not even then. “2x4” is just a nominal measurement. A construction grade 2x4 is cut rough from the tree, then planed down for a smooth finish which reduces its size down to the standard 2x4 dimension of roughly 1 1/2 x 3 1/2 inches.
Lumber is sold by the board/foot. This means you are paying for the shavings discarded when the board is planed down to size. Large mills sell these shavings, increasing their profits.
I recall working on shop I acquired built in 1949. The 2x4’s I bought in the 80’s matched the dimensions of the 2x4 framing on this shop building.