I saw this come up on social media and thought it would be appropriate to document here.  The question was asked "Okay; I try to search through posts and comments to find my answers; but I've tried to find this.... can anyone tell me what the four threaded holes are designed for on the inner quarter, right behind the door opening?"

Answer from @HS30-H, "Nothing to do with harness bars or harness attachments. They are the side mount hard points for the factory 'Safety Bar'/roll-over bar kit."

Photo of the holes:

 

And a pic showing the mount in use as intended:

 

 

From the yellow 1970 Nissan Sports 'Race & Rally Manual':

Thanks for sharing the diagram, Alan.

Aha, I knew we likely had a thread on this already.   Thanks Alan.

That diagram explains the deck mounts of the rear supports.  Love the diagram

Given the traditional reluctance of (mainstream) auto manufacturers to spend money on parts not vital for a functioning and legal-for-sale vehicle, it's interesting that someone within the Z's product design/engineering team was able to recommend and get approval for these competition-use-only attachment points to be installed in every one of the tens of thousands of cars that were going to be produced.  That's eight (maybe twelve?) captive nuts, individually tack-welded in place.

6 hours ago, Namerow said:

Given the traditional reluctance of (mainstream) auto manufacturers to spend money on parts not vital for a functioning and legal-for-sale vehicle, it's interesting that someone within the Z's product design/engineering team was able to recommend and get approval for these competition-use-only attachment points to be installed in every one of the tens of thousands of cars that were going to be produced.  That's eight (maybe twelve?) captive nuts, individually tack-welded in place.

There were four in the header rail adjacent to the interior light recess too. And then there's the captive nuts pre-installed in the radiator support for the factory oil cooler brackets and the holes in the radiator support crossmember for its 'feet'. There are also holes in the radiator support crossmember for the 432's air filter box and radiator overflow bottle. 

Lots of - shall we call them 'hermaphroditic'? - details to accommodate both LHD and RHD configurations too...