So this is what I found for the state of Iowa. Wonder why they made the cutoff 1948? Guess my 49 resembles a 48 so I'm going with that!
Registration plates issued for a motor vehicle which is model year 1948 or older, and
reconstructed or specially constructed vehicles built to resemble a model year 1948 vehicle
or older, other than a truck registered for more than five tons, autocycle, motorcycle, or truck
tractor, may display one registration plate on the rear of the vehicle if the other registration
plate issued to the vehicle is carried in the vehicle at all times when the vehicle is operated
on a public highway
