Just wondering when Datsun put the heavier impact rail inside the 240z doors? the car I bought had a heavy bar in the drivers door and a light weight on in the passenger. the two replacement doors I bought are the opposite. From a safety point I would like to have the heavy bar in both doors. Is it worth swapping the door skins? My car is dated 5-72. 

The difference in actual safety to the occupants with any of the S30 doors, is SO much less than any comparable, modern door, that it is not worth worrying about if you have the "best" doors or not.

Any car of this vintage requires that do all you can to actively practice defensive driving techniques at all times to avoid any form of accident in the first place. 

I believe they would have been added for the 1973 production year. It isn't listed specifically on zhome, but I would expect they were added at the same time as the other safety features. See here: http://zhome.com/History/DesignChanges.htm

Thank you both. I do agree with modern cars offering more protection. I don't think I will go thru all of the work to match them for the impact bars that very few will ever see. Also thank you for the link, it was interesting to read about the changes.