From what I was able to determine now:
From 1973 to 1989, an average points system was used, as in the number of yielded points was divided by the number of tournaments played. Bonus points were awarded for beating players ranked from X to Y.
From 1990 to 1999, a Best out of 14 system was used, Masters equivalent became mandatory along with GS and the average system was dropped. Bonus points were still awarded.
From 2000 to nowadays. Despite changes in the point counts, a Best out of 18 + WTF system was used, and the bonus points were dropped.
Now that I have the rules regarding all the systems used, I need to find out the values of ranking points for tournament categories for each year since 1973. Wikipedia is a bit helpful with the first years:http://en.wikipedia.org/wiki/1974_Grand_Prix_%28tennis%29
But I need to know all the values. Anyone can help here?