Hi GoGirl,
After the game is won, you hear a short tune played, then the box lid slides back, and you automatically come out of close-up. Does that happen for you?
Then straightaway you *should* see the open secret compartment and the key. If that isn't happening, you might have missed a number out somewhere? The puzzle allows you to win it after 3 failed tries.
I have a save game but my play doesn't follow the walkthrough, so you'd need to check against the tasklist for what has/hasn't been done so far. Let me know if you want that.
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"Bleat, Watson -- unmitigated bleat!" ~ Sherlock Holmes