How The Gauges Work

My prior digging around about such matters leads me to believe that the stabilized gauge design was originally invented by Smith Gauges out of England in the late nineteen-fifties or early sixties. That stabilizer design quickly proliferated, and by the mid-sixties pretty much all the auto manufacturers were using a version of that design.

A quick web search came up with lots of hits including this summary by whatever AI bot decided I needed to know:

"Smiths voltage stabilizers, used for their bimetallic gauges in classic cars (like MGs, Triumphs, Fords) from the 1960s onwards, were mechanical devices that converted varying dynamo/alternator voltage to a steady ~10V, preventing inaccurate fuel/temp readings; they worked by heating a bimetallic strip that rapidly opened/closed contacts"

So between that, and the fact that I've been inside a few mid-1970 gauges (which were stabilized just like the later ones), my answer to your question would be "I assume Datsun used the stabilized design from the very beginning."

As for the info in the service manual, I assume it's just another case of incomplete or incorrect info in the documentation. There are lots of examples of that.