Those wheels of yours have a hg freehub not a ug.That means you can use any 7 spd Shimano cassette,not just the older ug ones.
This modern stuff is a tad confusing at times.You will get to grips with it eventually.
I think Shimano still make a 7 spd cassette !!! I will check when I get to work.
^ Shaun, that one in your picture is definitely Uniglide and it'll take 6 or 7 gears. It just depends how many you decide to put on the splines and the lock cog just screws on a bit further for the 6. Hyperglide had an internal thread for the locknut, not an external one.