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I used the tweezers on the veil and removed the snake scale and received the 'ding' sounds that indicates the solving of a puzzle, but it didn't appear in my inventory. I read somewhere that I'm supposed to examine it again with the magnifying glass. Now Day 4 won't finish for me, and I think this is why.
So nobody has a clue? Oh well, no more Gabriel knight for me then :(
I haven't seen that problem before. But couldn't you just load an earlier save and try again to see if it works better next time?
I ran into the same problem before but I can't really remember how I solved it. I think I examined the tweezer to get the scale.

But at least you know that you didn't do anything wrong in the game. There's just an extra step you have to take.