The narrow slot I mentioned is right next to the wide tab.
Each sprocket has the tab to make the cassette line up correctly. The way the teeth designed in such away to aide is shifting. If you notice some look worn but this is done on purpose to make shifting smooth and this only works if they are oriented properly, hence the wide tab and narrower tooth.

You said you have a 7sp Ug on there, look at the hub face on and you should still be able to see the teeth on the FH body. Notice that there is a one tooth that is narrower than the others with a a wider slot to one side of it.