Can I assume that the point of this question is, if a club plans on doing a restoration, how do they go about selecting what point in their history represents the peak point to which the course should be restored?
Obviously, if a course was changed in the past, there were reasons that seemed valid at the time. And of course, sometimes a lot of changes were made at once but many other times little changes were made, one at a time, bit by bit. Behind each of those decisions was a human mind thinking "this needs to happen because......."
I suppose one way to go about determining a "high-water mark" would be to examine the major differences between the course as it is and the course as it was originally designed, and attempt to determine the thinking behind each of the changes, either through finding documentation or sheer guesswork, if necessary. It may turn out that there wasn't any one specific time where the entire course was at its best, but instead a time when each of the changes that have been made had their greatest validity, and that a restoration based on those decisions might not necessarily take a course back to one period in time but instead make the most of the course as it has existed over its entire history.
And who makes that determination? The people who always have - the chosen representatives of the members, both internal and external, yes?