one of only two online images I've ever seen with the temporary license plate number on a car

 https://www.curbsideclassic.com/vintage-snapshots-and-photography/vintage-snapshots-car-washing-in-the-50s-60s-2/

I found a Road Runner in the San Marino Classic car show of 2017, and learned that these were dealership temporary paper license plates.

https://justacarguy.blogspot.com/2015/06/a-68-road-runner-restored-to-level-ive.html