Shimano Uniglide

Lots of 80s bikes have a unique type of cassette that uses a threaded cog on the end to secure the gear cluster to the hub. This is called uniglide, and was Shimano's first attempt at making a cassette hub. These cassettes have twisted teeth, without cutouts or special shift ramps. Here is a very early 6 speed uniglide cassette hub from 1982:

 

Notice the threads on the outside of the freehub body. These are for the last cog which holds everything else in place. To remove the cassette you can use 2 chain whips to unscrew the smallest cog, or you can leave the wheel on the bike, shift into the biggest gear, and put the chain whip on the small cog. If you pedal forward with the chain whip secured in place, the cog will unscrew.

The last photo shows the complete cassette. Notice how there is only a dustcap at the front of the freehub body. This means that you can't install a modern hyperglide lockring to this freehub body. Some late 80s/early 90s hubs have threads for both cassette types. 

There are lots of ways to modify a uniglide cassette hub. The threads on a uniglide hub are the same as a standard freewheel, as well as a bottom bracket lockring. If you lose the smallest cog you can still use a bottom bracket lockring to hold the rest of the cassette to these hubs. You can also add a track cog to these threads to make a cheap singlespeed bike. Also keep in mind that uniglide cogs can be installed backwards if they wear out, thus doubling the life of the cassette.

Finally, you can install a newer hyperglide cassette to a uniglide hub if you grind off part of the wide spline on each gear (Hyperglide cassettes have one spline that is thicker than the rest in order to line each gear up with the shift ramps on adjacent gears. Uniglide cassettes have a uniform spline pattern). If your hub doesn't have the inside threads for a hyperglide lockring, you can just omit the smallest cog and use a bottom bracket lockring to secure the cassette. These hubs have lots of little quirks to them, but are definitely worth keeping around if you find a bike with one.