Phoenix’s Galactic Gallery wrote:
" I assume later attached to an overlay..the art itself as you see doesnt have words.
"
I do have pages from this era where the text was on a vellum overlay. However in that case usually there would be registration marks (a circle with a cross through it) taped to both the art page and the vellum page, so they could be lined up properly for reproduction. You can see an example here in the additional images, with Chris Claremont holding up the art and text pages separately, with the registration mark at the bottom of each.
I asked Tom Orzechowski how the text was added to the pages of X-Men 133 during production. He replied: "Most likely, the lettering, on vellum, was pasted onto photostats. As more and more books began to push the schedules, the practice moved to glueing directly onto the art. This allowed them to skip the step (and expense!) of making stats. How nice, meanwhile, that the art exists with all of Terry's background inks on display!"
And really, the art tells the story well enough that the text is superfluous. It is fun to see the marginal comments though.