I'm almost certain you are dealing with a Uniglide Cassette. These were the first version of Shimano's cassettes.
They require two chain whips to remove. One to hold the cassette and the other to unthread the top cog from the freehub body. Once the top cog is unthreaded, the whole unit slips off the freehub body.
Take a look here for the differences between freewheels, Uniglide, and Hyperglide.
Contact me via email and I might be able to help you out with a new cassette. (pastorbobnlnh "AT" gmail "DOT" com.