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:
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Москвичей предупредили о резком похолодании09:45
"At the end of the day, oil and gas companies have to deliver value to shareholders," he says. "They have very good managers. You can build anything, as long as you can pay for it.