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:
Teenage rocket maker
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第十四条 盲人或者又聋又哑的人违反治安管理的，可以从轻、减轻或者不予处罚。