Originally Posted by Wonder Boy
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This ad ran in 1978, promoting X-MEN as newly "On sale monthly!"
The last bi-monthly X-MEN was issue 111 (June 1978) and the first monthly was 112 (August 1978), and though Cockrum's last issue on interior stories was 107 (Oct 1977). It might be considered odd that Cockrum was selected to do the house ad.

But Cockrum still continued doing most covers for the series for another 2 years, through issue 126 (Oct 1979). At which point Cockrum resigned from Marvel, and whose resignation letter to Marvel oddly turned up "accidentally" as butler Jarvis' resignation letter in IRON MAN 127 (also Oct 1979), after a clash with Tony Stark in the alcoholism storyline issues.

Wow. I just ran across this ad in MICRONAUTS 21, Sept 1980, and other Marvel issues the same month. The same month that X-MEN 137 was published.
That's really late into the Byrne run, and way after Dave Cockrum was pencilling the series, or even doing covers.