Originally Posted by dddd

As I recall, the RSX cassette hub was from the beginning a Hyperglide-C (compact drive) -compatible freehub with stepped-down freehub body for 11t cogs.

As such, it should not have any external threading.

And at this late hour, few would be hoping for their cassette hub to be compatible with Uniglide cassettes, though I for one will use up those I have because of their friction-friendly shifting action.

Using a 6-speed cassette in a modern freehub is still entirely possible if a suitable lockring and spacers are used to compress the un-threaded cassette cogs, but a splined smallest cog would still need to be a part of the picture.