Have a look at this thread. Both my 12/70 and 6/71 cars had this tough, black coating applied to the bare metal before the bodies were sprayed with color.
Additionally, if you look very close at the pic showing the transmission tunnel in that thread, you will see a reddish over spray that came through the shifter opening onto the black coating. That overspray... is the first coat of primer that was applied at the factory. You can see it here on the fender I stripped back to bare metal:
Over this red "oxide" primer, which was sprayed over the entire top surface of the car, they sprayed a coat of grey primer before the paint:
Note that this picture is of the front wheel well area where the cowl drain hose passes from the inside of the passenger compartment of the car to the outside - left hand side of the car. We are looking at the place where the fender support panel bolts to the unibody. To the left of where the support was bolted, you can see a "black" coating in the picture. To the right of where the support was bolted, you see no black coating. The support panel also had the black coating on it.
The same is true for the front fender. The black coating is only on the inside of the fender, in front of where the support panel contacts the back of the fender.
Those last facts are a bit off point, but circling back the thread for which I provided a link, you can clearly see the silver is on top of the black, and the black is on top of bare metal. All of this is what I provide as facts that the black coating was applied at the Nissan factory. I also have chips of the black coating which flaked off in some places. And I have sanded it in various places. So, I know it is the same stuff all over the car. No other black substance was sprayed on this car (6/71) after the original silver paint.