This: https://i.imgur.com/rjfxoRy.gif

The pressed state for the close box was a sort of starburst thing, not an X. (Possibly you’re thinking of checkboxes, which were squares that got crossed out when they were checked.)

I'm not sure if there's a name for it, but it's not an X. You can try it yourself here: https://infinitemac.org/1988/System%206.0

The “x” during hover was apparently added in MacOS X.