1965 Dodge Dart Charger

65dartcharger said
Did the dealership mount the license plate for his customer

No, but in a 2-plate state the pre-sale prep usually included installing the front plate mounting hardware to make it ready for the buyer to install his plate.

When the first owner of the car sold it, did he keep the plates then?

Usually so. Whether or not he could then use them on another car varies by state.