Question: After which Olympic year was the 40mm ball introduced?
About the article "Table tennis", section "Ball".

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The 7.62×40mm Wilson Tactical (7.62×40mm WT) is a centerfire rifle cartridge introduced in 2011 by Wilson Combat.
The goal was to produce an accurate, low-recoil .30-caliber cartridge that could be used in AR-15/M4 rifles using as many standard components as possible. 
The SMC Pentax-DA 40mm f/2.8 Limited lens is a pancake lens for the Pentax K-mount.
At only 0.6 in (15 mm) long, it was the smallest lens made by Pentax until the 40mm XS lens was introduced.
The lens has a 35mm equivalent focal length of 60mm; it is part of the Limited series of high-performance compact lenses. 
The international rules specify that the game is played with a light 2.7 gram, 40 mm diameter ball.
The rules say that the ball shall bounce up 24–26 cm when dropped from a height of 30.5 cm onto a standard steel block thereby having a coefficient of restitution of 0.89 to 0.92.
The 40 mm ball was introduced after the 2000 Olympic Games.
However, this created some controversy as the Chinese National Team argued that this was merely to give non-Chinese players a better chance of winning since the new type of balls has a slower speed, while at that time most Chinese players were playing with fast attack and smashes.
A 40 mm table tennis ball is slower and spins less than the original 38 mm (1.5 inch) one.
The ball is made of a high-bouncing air-filled celluloid or similar plastics material, colored white or orange, with a matte finish.
The choice of ball color is made according to the table color and its surroundings.
For example, a white ball is easier to see on a green or blue table than it is on a gray table.
Some Table Tennis balls have ball rated in stars so that more professional players can know which ball to use.
Manufacturers often indicate the quality of the ball with a star-rating system, usually from one to three, three being the highest grade.
As this system is not standard across manufacturers, the only way a ball may be used in official competition is upon ITTF approval.
The ITTF approval can be seen printed on the ball.
The Milkor 37/38mm Less Lethal “Stopper” Single-shot was designed and distributed in 1981 by Milkor (Pty) Ltd as a less-lethal anti-riot weapon along with the Milkor 40mm Single shot Grenade Launcher which was designed as a reduced-cost grenade launcher for the SWAPOL Forces.
In 2008, Milkor (Pty) Ltd released a new design designated the Milkor 37/38mm or 40mm Stopper Convertible, which is a single shot break-open weapon designed to utilize a variety of 37/38mm or 40mm Less Lethal rounds.
As with the original Milkor MGL, the Stopper was originally marketed for Milkor by Armscor (South Africa). 
In 1936, because he was Jewish, Ball (the 25-year-old captain) was initially overlooked for selection in the German ice hockey team.
His good friend and teammate, Gustav Jaenecke, refused to play unless Ball was included.
Ball also believed a deal could be struck to save his family in Germany if he returned to play in the games.
The German selectors also realized that without Ball and Jaenecke the team would not stand a chance of winning.
Another factor was that the Nazi party could not overlook the fact that Ball was without doubt one of the leading athletes in his sport.
With much controversy Ball was included in the German team to play at the 1936 Olympic games.
One report of the time proposed that Ball was playing against his will.
The deal for Ball's family to leave Germany was also agreed.
After Ball was injured, the Germans took 5th place in the Olympic tournament.
Ball played four matches and scored two goals.
Ball followed his brother, Heinz, to South Africa in 1948.
He died in Johannesburg in 1975.