What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
Москвичей предупредили о резком похолодании09:45
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Медведев вышел в финал турнира в Дубае17:59
RadialB's fake videos portray grimy Croydon waterparks and an arcade machine filled with knives
The false positives at the bottom