The cassette in question in a Uniglide, which as you would have found out, is held on by the last cog, which is threaded.
The hub is a Uniglide-only hub; you will notice that there is no provision for a Hyperglide threaded lockring in the freehub body.
My preferred option would be to upgrade the freehub body to a 7 speed Hyperglide variety, which would allow for a better choice of cassette, as HG cassettes are easier to come by than UG. The freehub body is removed with a 10mm Allen key from inside the hub. You do this by removing the axle assembly, which then allows access to the internals. The only problem you may encounter when doing a swap is incompatibility between the new freehub body's bearing race and the existing axle's cone. However, more often than not it works, so it is worth the effort. I don't know what you mechanical abilities are in terms of servicing hubs etc., but Sheldon' Brown's site is a good reference if you get confused.
If this seems too difficult, then obviously just buying the UG cassette from Loose Screws is a better option. In either case, make sure you get a chain to go with it.