I mean, it could be a manual photoshop job.
It could, but the double spiral in the shell indicates AI to me. Snail shells don't grow like that. If it was a manual job, they would have used a picture of a real shell.
Edit: plus the cat head looks weird where it connects to the head, and the markings don't look right to me.
Byrne began drawing X-men in 1977 (Uncanny X-men #108), and the brown suit was introduced in #139 (nov 1980) so 70s is definitely your best bet here. (This is 100% a Byrne panel.)
Edit: I had a hunch and found the issue, it's from #125 september 1979, page 6 ("The perils of the Danger Room!").