I couldn't find the recipe either, but I found the weapon itself for sale at the auction house. So I bought one and used it to make the enhanced hard wooden bow.
The only thing I can figure is that even though the recipe is not available, the bow is available somewhere, and can then be used as an ingredient for the enhanced version... I haven't checked the weapons vendors to see if you can buy the hard wooden bow from one of them. Being that it's only a lvl 11, I'd guess that if a weapon vendor sells it, you would find it in Logar or maybe Varanas. If not, then I'd guess the bow is an item that gets dropped in combat.
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