10 speed cassettes are narrower than 9 speed so you can't fit a 9 speed cassette (or 11, or anything else), and the taller splines may prevent you doing do anyway - and also block some 10 speed cassettes.
Thankfully rare and only found on a handful of Shimano wheelsets AFAIK.
(NAM) What kind of freehub body is this? Old school Shimano?
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