I think early Shimano cassettes had a threaded top sprocket which acted as the lockring too. The resultant threading on the freehub body seems to have persisted (to some degree) ever since.
@Bearlegged started
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I think early Shimano cassettes had a threaded top sprocket which acted as the lockring too. The resultant threading on the freehub body seems to have persisted (to some degree) ever since.