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 https://infinitemac.org/1988/S

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 https://infinitemac.org/1988/System%206.0 The “x” during hover was apparently added in MacOS X.