I am not sure if there is such a thing as exact location. You may notice that, if you put the passenger mirror in the exact spot as the driver's mirror, you would not be able to see clearly (the mirror would be just a decoration). MY understanding is that these mirrors in 60-ies were put on by dealers (much like antennas, radios and A/Cs), so finding exact position may be a moot point.
If you would want the mirror to be of any use, you may want to have someone hold the mirror on the door, while you sit at the drivers seat and move it slightly until it you can see the back of the car correctly and all makes sense and it proves to be usable. Then mark on the door, drill and fasten. You would want to fasten the mirror with the enforcement plate position underneath (inside of the door) before fastening. This will help to prevent any denting on the aluminium door when accidentally leaning against the mirror.
If you cannot find the enforcement plate, try searching usual vendors for pagoda parts.