I've never before seen a Yanagisawa sterling silver alto neck with the normal octave key. Many of the sterling tenor necks have that feature because there's an extra brass brace under the neck, which would get in the way of the underslung octave key. Presumably, this is because the silver is a bit soft, and the brace is needed to help prevent a pull-down.
In any event, your neck has a general kind of 80s/90s look to it, but that's just a guess.
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