Ron
I'll repeat my question from the discussion we had on this subject some 2 years ago. It wasn't answered then. Perhaps it will be this time.

If the reason for the exposed gap at the head of a dry fly is to accommodate the turle knot, why is it not found on wet flies and nymphs from the same tiers during the same time period?

Yes, it's trick question. No, I don't know the answer with certainty.