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!”).
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!”).
Whew, that was close! I knew something about Wolverine’s hairless arms were disco