Sounds like its likely a uniglide freehub. Most 6 or 7 speed (i think the 7 should be ok)uniglide cassettes should work (if you can find one, they dont make them anymore), be careful with dura ace uniglide stuff as i think the splines on the freehub were different.
If you have indexing for 6 speed obviously it wont work with 7, only in friction mode. There are other alternatives like replacing the freehub, preferably to a 126mm 7 speed one to maintain correct wheel dish. But again, you'd have to find one. (Think they only made them briefly for 600, 105 and rx100 - they have both internal and external threads for accepting uniglide and hyperglide cassettes.
Easiest/cheapest solution might be to get a 7 speed hyperglide cassette (the cogs/sprockets are the same width as 6 speed i beleive, just the spacers are thinner) and dismantle both and use say the top 1 or 2 hyperglide sprockets to get a decent ratio cassette. To do this however you would need to file or grind off the wide spline on the sprockets you wish to fit.
This should be a suitable donor cassette ending soon!
http://cgi.ebay.co.uk/Shimano-7-Speed-C ... 2a113367dd