The front passenger side antenna was standard with the AM and the AM/FM radio. The rear passenger side antenna was optional with the AM radio. Anything other than that was not factory installed, so either dealer installed or from a prior owner. I have (IIRC) two 68's in the database with the antenna on the front driver side. This is the first one I've ever seen with the antenna on the rear driver side.
Remember, dealers were free to do whatever they wanted to do. The factory wasn't. And this includes Fisher body (the folks that installed the rear antenna).
Ed