Only one option for factory antenna on '65 A-body: 2291 336. Pretty sure they were not made by Motorola. Lots of cars of that era wound up with random antennas on them because often a radio was not added until after the car was built and shipped.
Anybody have details of the factory field installation kits for radios? I'm sure they would have included an antenna. Would it have been that p/n? I suspect other than having a Chrysler specific style of base, any antenna vendor would have been free to use their own mast design as long as it passed Chrysler tech requirements.