Question: When was the Mona Lisa fitted?
About the article "Mona Lisa", section "Frame".

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Because the "Mona Lisa"'s poplar support expands and contracts with changes in humidity, the picture has experienced some warping.
In response to warping and swelling experienced during its storage during World War II, and to prepare the picture for an exhibit to honor the anniversary of Leonardo's 500th birthday, the "Mona Lisa" was fitted in 1951 with a flexible oak frame with beech crosspieces.
This flexible frame, which is used in addition to the decorative frame described below, exerts pressure on the panel to keep it from warping further.
In 1970, the beech crosspieces were switched to maple after it was found that the beechwood had been infested with insects.
In 2004–2005, a conservation and study team replaced the maple crosspieces with sycamore ones, and an additional metal crosspiece was added for scientific measurement of the panel's warp.
The "Mona Lisa" has had many different decorative frames in its history, owing to changes in taste over the centuries.
In 1909, the Comtesse de Béhague gave the portrait its current frame, a Renaissance-era work consistent with the historical period of the "Mona Lisa".
The edges of the painting have been trimmed at least once in its history to fit the picture into various frames, but no part of the original paint layer has been trimmed.
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By the 20th century, "Mona Lisa" had already been a victim of satirical embellishment.
"Sapeck" (Eugène Bataille), in 1883, depicted "Mona Lisa" smoking a pipe ("shown above").
Titled "Le Rire" (The Laugh), the artwork was displayed at the "Incoherents" exhibition in Paris at the time of its creation, making it among the earliest known instances of "Mona Lisa"'s image being re-interpreted using contemporary irony.
Further interpretations by avante garde artists beginning in the early 20th century, coinciding with the artwork's theft, attest to "Mona Lisa"'s popularity as an irresistible target.
Dadaists and Surrealists were quick to modify, embellish and caricature "Mona Lisa"'s visage. 
The Mona Lisa is an alien organism that exists in a mineral that was used in paint supplied by Giuseppe Di Cattivo in the 15th century.
Leonardo da Vinci used the paint for his famous Mona Lisa, and when the picture is brought into proximity with Giuseppe's notorious painting of The Abomination - a painting considered too terrifying to be seen - both the pictures come to life in "Mona Lisa's Revenge".
In order to release The Abomination, she forced Clyde to draw a key that she could use to turn the key to reality.
The Mona Lisa was successful in doing so, but accidentally transforms a previous drawing of K-9 to life as well, who locks The Abomination and return all moving paintings to its original positions.