On the ATP website, the weekly update of the rankings started only on the 25th of june 1984.
Before that, except in cases when the situation was 100% clear (a player being very much ahead of the others), it seems impossible to know an exact number of weeks at this or that rank, you can only get a maximum if the case was optimal for him because he could always have lost a rank for a few weeks if a weekly update had been made.
Now Fed has 300 weeks guaranteed, 302 if Djokovic doesn't ask a wild-card in Basel or in tournaments next week.
I second this statement, although I did mention this before. Fed's 300 is also official, because it is backed up by solid data. Before 1973 when the computer rankings were introduced, the #1 status was an unclear vortex of collected data, nothing official, nothing certain. It's exactly like the world population approximation before recension was introduced. Nobody knows the exact numbers, just estimates, but the error chance is high.
Are you guys sure? I see on the wikipedia list that Borg became number 1 on august 23rd 1977 and held that position for just 1 week.