This is Becker Website
http://www.beckerautosound.com
I believe all the manual antennas were install on the left side of the car while all car with power atennas were install on the right side of the car.
Had a 74 911 Coupe with manual antenna on the left side
Current 82 911 SC Targa has a power antenna on the right side.
One reason that Porsche may had move the antenna to the right side is that the drier for the a/c unit is located on the left side and most Porsche 911 shipped to the U.S. came with a/c, unless it was order with the a/c delete option.
I don't think even a OEM power antenna will fit the left side. At one time someone made a power antenna were the motor could be mounted remotely (2') from the mast. It was designed for location with limited access. Check the J.C. Whitney Catalog they may still have it.