The oldest stats on page 717 of The Proclaimes book go back to the 1920's, but I guess they must have had them earlier. I thought this comment was interesting :
The method of counting publishers underwent a number of changes in the 1920'sand the early 1930's.Congregation reports were sent to the Society weekly instead of once amonth. (Monthly reports did not go into effect until October 1932) To be counted as a Class Worker (Congregation Publisher) one had to devote at least three hours a week (or twelve per month) to the field service , according to "The Bulletin" of January 1st 1929. Sharpshooters (isolated puplishers)were to devote at least two hours per week to witnessing.
Phew!!