I am desperately looking for a 7 speed cassette that would fit on an old shimano
600 hub. This is the kind that the small cog screws onto the end of the hub assembly.
The hub has threads on the outside of the assembly unlike all the new ones that
are internally threaded. The owner of this bike wore the thing out!
Riding: 1960s Falcon commuter; Queued: 1977 Bob Jackson, 1983 Serotta Club Special, 1984 Motobécane Team Champion, 1983 Guerciotti SLX, 1974 Harding (like Holdsworth Pro), 1974 Peugeot PX10LE, 1970s Jeunet Franche-Comté, 1974 Raleigh International
disclaimer: this is lore gleaned from the Internet, not from bonafide personal experience
If you can find a Hyperglide cassette that works for you, and take it apart, I think that a bit of grinding is all you need to turn it into a Uniglide cassette. The difference is that the H-glide sprockets all have one spline that's wider than the rest (and he freehub body has a corresponding narrow spline), so that there's only one way (rotationally speaking) the sprockets can go together. This ensures that the pre-engineered tooth transitions for smoother shifting are in the right places.
Obviously, if you do this, you should mark each sprocket when you take it apart, so that the cassette goes back together in the same orientation. Use a dremel or other grinder tool to narrow the widest spline (one) in each sprocket and spacer, being careful to do this so that all the splines and slots are basically the same, as on the Uniglide hub. Then reassemble, with all the narrowed splines in alignment with each other, to preserve the tooth engineering.
Some Hyperglide cassettes will be held together with machine screws, and others have rivets. Obviously the rivets would need to be cut to get the cassette apart, and replaced with bolts. Also, some cassettes have a unitized "spider" construction for the larger sprockets, which may make disassembly of those difficult or impossible (see SB page, and do a word search for "spider," repeat).