Yeah, my mistake. A sharpie would be used for inking, and all the X-MEN pages and covers I linked above were Byrne pencils, none showed Byrne inks.

But the Rog 2000 pages I showed are Byrne pencils and inks.

And virtually all the art in the ART OF JOHN BYRNE book from my opening post (published in 1980) are examples of Byrne's ability with a pen, if not a Sharpie pen in particular.




And on second glance, you're right, issue 143 was Austin pencils/Austin inks(no Byrne). But it sure looks like Byrne art.
Issue 142's cover as well (a futuristic Sentinel killing all the X-Men) is Austin pencils and inks, with no Byrne art.

So that would make Byrne's spectacular cover for issue 141 his last X-MEN series cover.

[Click images to enlarge]

But as you say, while very pretty, none of these X-MEN pages shows Byrne inks, only his pencils.