This is verbatim current federal law (49 CFR 565), I'm sure it was similar in 1968- "The VIN for passenger cars, multipurpose passenger vehicles, low speed vehicles, and trucks of 4536 kg or less GVWR shall be located inside the passenger compartment. It shall be readable, without moving any part of the vehicle, through the vehicle glazing under daylight lighting conditions by an observer having 20/20 vision (Snellen) whose eye-point is located outside the vehicle adjacent to the left windshield pillar."
Basically says all cars must have a clearly visible VIN seen through the driver's side windshield. Federal law supersedes state law.
